Tag Archives: c#

Adding Logging to Client Side Blazor

Whilst there are some pre-built libraries for this, I seemed to be getting Mono linking errors. What finally worked for me was to install the pre-release versions of:

Install-Package Microsoft.Extensions.Logging -Version 3.0.0-preview6.19304.6
Install-Package Microsoft.Extensions.Logging.Console -Version 3.0.0-preview6.19304.6

Now, in your View Model, accept the Logger:

public MyViewModel(ILogger<MyViewModel> logger)

Then you can log as normal:

_logger.LogInformation("Hello, here's a log message");

You should now see the debug message in the F12 console.

You might be wondering why you don’t need to explicitly inject the logging capability; the reason is that:

BlazorWebAssemblyHost.CreateDefaultBuilder()            

Does that for you.

Casting a C# Object From its Parent

Have you ever tried to do something akin to the following:

[Fact]
public void ConvertClassToSubClass_Converts()
{
    // Arrange
    var parentClass = new SimpleTestClass();
    parentClass.Property1 = "test";
 
    // Act
    var childClass = parentClass as SimpleTestSubClass;
 
    // Assert
    Assert.Equal("test", childClass.Property1);
}

This is a simple Xunit (failing) test. The reason is fails is because you (or I) am trying to cast a general type to a specific, and C# is complaining that this may not be possible; consequently, you will get null (or for a hard cast, you’ll get an InvalidCastException.

Okay, that makes sense. After all, parentClass could actually be a SimpleTestSubClass2 and, as a result, C# is being safe because there’s (presumably – I don’t work for MS) too many possibilities for edge cases.

This is, however, a solvable problem; there are a few ways to do it, but you can simply use reflection:

public TNewClass CastAsClass<TNewClass>() where TNewClass : class
{
 
    var newObject = Activator.CreateInstance<TNewClass>();
    var newProps = typeof(TNewClass).GetProperties();
 
    foreach (var prop in newProps)
    {
        if (!prop.CanWrite) continue;
 
        var existingPropertyInfo = typeof(TExistingClass).GetProperty(prop.Name);
        if (existingPropertyInfo == null || !existingPropertyInfo.CanRead) continue;
        var value = existingPropertyInfo.GetValue(_existingClass);
        
        prop.SetValue(newObject, value, null);
    }
 
    return newObject;
}

This code will effectively transfer any class over to any other class.

If you’d rather use an existing library, you can always use this one. It’s also Open Source on Git Hib.

The Evolution of the Switch Statement (C#8)

Most languages have a version of the switch statement as far as I’m aware; I must admit, I don’t remember one from Spectrum Basic, but ever since then, I don’t think I’ve come across a language that doesn’t have one. The switch statement in C was interesting. For example, the following was totally valid:

switch (value)
{
    case 1:
        printf("hello ");
    case 2:
        printf("world");        
}

If you gave it a value of 1 would print “hello world”. When C# came out, they insisted on using breaks at the end of case statements, or having no code (admittedly there were a few bugs in C caused by accidentally leaving break statements out):

            int value = 1;
            switch (value)
            {
                case 1:
                    Console.Write("hello ");
                    break;
                case 2:
                    Console.Write("world");
                    break;
            }

Anyway, fast forward around 17 years to C# 7.x, and it basically has the same switch statement; in fact, as far as I’m aware, you could write this switch statement in C# 1.1 and it would compile fine. There’s nothing wrong with it, so I imagine MS were thinking why fix it if it’s not broken.

There are limitations, however; for example, what if I want to return the string, like this:

            int value = 1;
            string greeting = string.Empty;
            switch (value)
            {
                case 1:
                    greeting = "hello ";
                    break;
                case 2:
                    greeting = "world";
                    break;
            }

            Console.WriteLine(greeting);

Now it looks a bit cumbersome. What if we could write it like this:

            int value = 1;
            string greeting = value switch
            {
                1 => "hello ",
                2 => "world",
                _ => string.Empty
            };

            Console.WriteLine(greeting);

From C# 8, you can do just that. The switch statement will return its value. The case syntax is disposed of, and there’s no need for a break statement (which, to be fair, can encourage people to write large swathes of code inside the switch statement – if you don’t believe me then have a look in the Asp.Net Core source!).

And that’s not all. Pattern matching has also been brought in; for example, take the following simple class structure:

    interface IAnimal
    {
        void Eat();
        void Sleep();            
        string Name { get;}
    }

    class Dog : IAnimal
    {
        public string Name { get => "Fido"; }

        public void Eat()
        {
            Console.WriteLine("Dog Eats");
        }

        public void Sleep()
        {
            Console.WriteLine("Dog Sleeps");
        }
    }

    class Cat : IAnimal
    {
        public string Name { get => "Lemmy"; }

        public void Eat()
        {
            Console.WriteLine("Cat Eats");
        }

        public void Sleep()
        {
            Console.WriteLine("Cat Sleeps");
        }
    }

We can put that into a switch statement like this:

            IAnimal animal = new Cat();
            string greeting = animal switch
            {
                Dog d => $"hello dog {d.Name}",
                Cat c => $"hello cat {c.Name}",                
                _ => string.Empty
            };

            Console.WriteLine(greeting);

We can actually do better that this (obviously better is a relative term). Let’s say that we wanted to do something specific for our particular cat:

            IAnimal animal = new Cat();
            string greeting = animal switch
            {
                Dog d => $"hello dog {d.Name}",
                Cat c when c.Name == "Lemmy" => $"Hello motorcat!",
                Cat c => $"hello cat {c.Name}",                
                _ => string.Empty
            };

            Console.WriteLine(greeting);

It’s a bit of a silly and contrived example, but it does illustrate the point; further, if you switch the case statements around for the general and specific form of Cat, you’ll get a compile error!

ReadOnly Entity Framework

Ever come across a situation where you want to use an Entity Framework data layer in a little tool or report, but ended up reverting to Ado.Net or a different ORM because EF is a little bit like a Heisenberg Framework (it changes things that it looks at)? You might have a seed method that tries to update data, or a migration that you don’t want to apply.

Apologies in advance if this seems like stating the bloody obvious; but it didn’t seem obvious to me at the time. If there’s a neater way to do this then please let me know.

Anyway, I recently had this issue, and came up with the idea of a ReadOnly DBContext. For example, your main DBContext might look like this:

    public class MyDbContext : DbContext, IMyDbContext
    {
        static MyDbContext()
        {            
            Database.SetInitializer(new MigrateDatabaseToLatestVersion<MyDbContext, Configuration>());
        }
 
        public MyDbContext()
            : base("connection-string")
        {
            
            
        }
 
        public MyDbContext(string connectionString)
            : base(connectionString)
        {
 
 
        }
 
        public DbSet<Entity1> Entity1 { get; set; }
        public DbSet<Entity2> Entity2 { get; set; }
 
        void IMyDbContext.SaveChanges()
        {
            base.SaveChanges();
        }
 
        Task IMyDbContext.SaveChangesAsync()
        {
            return base.SaveChangesAsync();
        }
 
        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Entity1>()
                .HasOptional(a => a.Field2)
                .WithMany();            
  
            base.OnModelCreating(modelBuilder);
        }
    }

Making a ReadOnly version of this is pretty much removing the code; for example:

    public class MyReadOnlyDbContext : DbContext, IMyDbContext
    {
        static MyReadOnlyDbContext()
        {            
            Database.SetInitializer(new MigrateDatabaseToLatestVersion<MyReadOnlyDbContext, Configuration>());
        }
 
        public MyReadOnlyDbContext()
            : base("connection-string")
        {
            
            
        }
 
        public MyReadOnlyDbContext(string connectionString)
            : base(connectionString)
        {
 
 
        }
 
        public DbSet<Entity1> Entity1 { get; set; }
        public DbSet<Entity2> Entity2 { get; set; }
 
        void IMyDbContext.SaveChanges()
        {

        }
 
        Task IMyDbContext.SaveChangesAsync()
        {
		  return Task.CompletedTask;

        }
 
        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Entity1>()
                .HasOptional(a => a.Field2)
                .WithMany();
  
            base.OnModelCreating(modelBuilder);
        }
    }

An easy way to return the read only version, rather than the main version is via a DbContextGenerator:

public class DbContextGenerator : IDbContextGenerator
{
    private readonly AppSettings appSettings;        
 
    public DbContextGenerator(AppSettings appSettings)
    {
        this.appSettings = appSettings;            
    }
 
    public IMyDbContext GenerateDbContext()
    {            
        if (appSettings.ReadOnly)
        {
            return new MyDbReadOnlyContext(appSettings.DatabaseConnectionString);
        }
        return new MyDbContext(appSettings.DatabaseConnectionString);                        
    }
}

This can be registered in the IoC and will return a DB Context based on a read only flag.

It’s worth noting that if your model doesn’t match, it will likely crash. It’s also worth noting that if you’re reading this article and trying to work out how you can use EF without having it manage the DB then it’s probably because something, somewhere, went wrong: this isn’t a suggestion for a new way to use EF, it’s a way to use EF in a temporary fashion.

Asp.Net Core 2.0 – Passing data into a Model Using DI

Imagine you have an Asp.Net Core web page, and you would like to edit some data in a form, but you’d like to default that data to something (maybe initially specified in the Web.Config).

I’m sure there’s dozens of ways to achieve this; but this is one.

Let’s start with a bog standard MVC web app:

Step one is to define a model in which to hold your data; here’s mine:

public class CurrentAppData
{
    public string DataField1 { get; set; }
}

Let’s register that in the IoC container:

// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
    services.Configure<CookiePolicyOptions>(options =>
    {
        // This lambda determines whether user consent for non-essential cookies is needed for a given request.
        options.CheckConsentNeeded = context => true;
        options.MinimumSameSitePolicy = SameSiteMode.None;
    });
 
 
    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
 
    services.AddTransient<CurrentAppData, CurrentAppData>(
        a => new CurrentAppData()
        {
            DataField1 = "test value"
        });

Next thing we’ll need is a View, and a corresponding view model to edit our data; here’s the view:

@model EditDataViewModel
@{
    ViewData["Title"] = "Edit Data";
}
 
<h2>@ViewData["Title"]</h2>
 
<div>
    <label>Change data here:</label>
    <input type="text" asp-for="EditData.DataField1" />
 
</div>

And now the view model (that is, the model that is bound to the view):

public class EditDataViewModel
{
    public EditDataViewModel(CurrentAppData editData)
    {
        EditData = editData;
    }
    public CurrentAppData EditData { get; set; }
}

The final step here is to adapt the controller so that the CurrentAppData object is passed through the controller:

public class EditDataController : Controller
{
    private readonly CurrentAppData _currentAppData;
 
    public EditDataController(CurrentAppData currentAppData)
    {
        _currentAppData = currentAppData;
    }
 
    public IActionResult EditData()
    {
        return View(new EditDataViewModel(_currentAppData));
 
 
    }
}

That works as far as it goes, and we now have the data displayed on the screen:

The next step is to post the edited data back to the controller; let’s change the HTML slightly:

<form asp-action="UpdateData" asp-controller="EditData" method="post" enctype="multipart/form-data">
    <label>Change data here:</label>
    <input type="text" asp-for="EditData.DataField1" />
    <br />
    <button type="submit" class="btn-default">Submit Changes</button>
</form>

We’ve added a submit button, which should cause the surrounding form element to execute whichever “method” it’s been told to (in this case, POST). It will look for an action on the controller called UpdateData, so we should create one:

public IActionResult UpdateData(EditDataViewModel editDataViewModel)
{
    System.Diagnostics.Debug.WriteLine(editDataViewModel.EditData.DataField1);
    return View("EditData", editDataViewModel);
}

Here, we’re accepting the EditDataViewModel from the view. However; when we run this, we get the following error:

Error:

InvalidOperationException: Could not create an instance of type ‘WebApplication14.Models.EditDataViewModel’. Model bound complex types must not be abstract or value types and must have a parameterless constructor. Alternatively, give the ‘editDataViewModel’ parameter a non-null default value.

Let’s first implement a fix for this, and then go into the whys and wherefores. The fix is actually quite straightforward; simply give the view model a parameterless constructor:

public class EditDataViewModel
{
    public EditDataViewModel()
    {
        
    }
 
    public EditDataViewModel(CurrentAppData editData)
    {
        EditData = editData;
    }
    public CurrentAppData EditData { get; set; }
}

The problem that we had here is that the `EditDataViewModel` that is returned to UpdateData is a new instance of the model. We can prove this by changing our code slightly:

Here, we’ve added a field called TestField1 to the model, and populated it just before we pass the model to the view; and on the post back, it’s gone. I’m not completely sure why the view model can’t be created by the middleware in the same way that the controller is; but that’s the subject of another post.

Finally, show the value back to the screen

To wrap up, we’ll just show the same value back to the screen; we’ll add an extra value to the model:

public class CurrentAppData
{
    public string DataField1 { get; set; }
 
    public string DisplayField1 { get; set; }
}

And we’ll just display it in the view:

<form asp-action="UpdateData" asp-controller="EditData" method="post" enctype="multipart/form-data">
    <label>Change data here:</label>
    <input type="text" asp-for="EditData.DataField1" />
    <br />
    <button type="submit" class="btn-default">Submit Changes</button>
    <br />
    <label>@Model.EditData.DisplayField1</label>
    <br />
</form>

Finally, we’ll copy that value inside the controller (obviously, this is simulating something meaningful happening), and then display the result:

public IActionResult UpdateData(EditDataViewModel editDataViewModel)
{
    editDataViewModel.EditData.DisplayField1 = editDataViewModel.EditData.DataField1;
    return View("EditData", editDataViewModel);
}

Let’s see what that looks like:

Using NSubstitute for partial mocks

I have previously written about how to, effectively, subclass using Nsubstitute; in this post, I’ll cover how to partially mock out that class.

Before I get into the solution; what follows is a workaround to allow badly written, or legacy code to be tested without refactoring. If you’re reading this and thinking you need this solution then my suggestion would be to refactor and use some form of dependency injection. However, for various reasons, that’s not always possible (hence this post).

Here’s our class to test:

public class MyFunkyClass
{
    public virtual void MethodOne()
    {        
        throw new Exception("I do some direct DB access");
    }
 
    public virtual int MethodTwo()
    {
        throw new Exception("I do some direct DB access and return a number");

        return new Random().Next(5);
    }
 
    public virtual int MethodThree()
    {
        MethodOne();
        if (MethodTwo() <= 3)
        {
            return 1;
        }
 
        return 2;
    }
}

The problem

Okay, so let’s write our first test:

[Fact]
public void Test1()
{
    // Arrange
    MyFunkyClass myFunkyClass = new MyFunkyClass();
 
    // Act
    int result = myFunkyClass.MethodThree();
 
    // Assert
    Assert.Equal(2, result);
}

So, what’s wrong with that?

Well, we have some (simulated) DB access, so the code will error.

Not the but a solution

The first thing to do here is to mock out MethodOne(), as it has (pseudo) DB access:

[Fact]
public void Test1()
{
    // Arrange
    MyFunkyClass myFunkyClass = Substitute.ForPartsOf<MyFunkyClass>();
    myFunkyClass.When(a => a.MethodOne()).DoNotCallBase();
 
    // Act
    int result = myFunkyClass.MethodThree();
 
    // Assert
    Assert.Equal(2, result);
}

Running this test now will fail with:

Message: System.Exception : I do some direct DB access and return a number

We’re past the first hurdle. We can presumably do the same thing for MethodTwo:

[Fact]
public void Test1()
{
    // Arrange
    MyFunkyClass myFunkyClass = Substitute.ForPartsOf<MyFunkyClass>();
    myFunkyClass.When(a => a.MethodOne()).DoNotCallBase();
    myFunkyClass.When(a => a.MethodTwo()).DoNotCallBase();
 
    // Act
    int result = myFunkyClass.MethodThree();
 
    // Assert
    Assert.Equal(2, result);
}

Now when we run the code, the test still fails, but it no longer accesses the DB:

Message: Assert.Equal() Failure
Expected: 2
Actual: 1

The problem here is that, even though we don’t want MethodTwo to execute, we do want it to return a predefined result. Once we’ve told it not to call the base method, you can then tell it to return whatever we choose (there are separate events – see the bottom of this post for a more detailed explanation of why); for example:

[Fact]
public void Test1()
{
    // Arrange
    MyFunkyClass myFunkyClass = Substitute.ForPartsOf<MyFunkyClass>();
    myFunkyClass.When(a => a.MethodOne()).DoNotCallBase();
    myFunkyClass.When(a => a.MethodTwo()).DoNotCallBase();
    myFunkyClass.MethodTwo().Returns(5);
 
    // Act
    int result = myFunkyClass.MethodThree();
 
    // Assert
    Assert.Equal(2, result);
}

And now the test passes.

TLDR – What is this actually doing?

To understand this better; we could do this entire process manually. Only when you’ve felt the pain of a manual mock, can you really see what mocking frameworks such as NSubtitute are doing for us.

Let’s assume that we don’t have a mocking framework at all, but that we still want to test MethodThree() above. One approach that we could take is to subclass MyFunkyClass, and then test that subclass:

Here’s what that might look like:

class MyFunkyClassTest : MyFunkyClass
{
    public override void MethodOne()
    {
        //base.MethodOne();
    }
 
    public override int MethodTwo()
    {
        //return base.MethodTwo();
        return 5;
    }
}

As you can see, now that we’ve subclassed MyFunkyClass, we can override the behaviour of the relevant virtual methods.

In the case of MethodOne, we’ve effectively issued a DoNotCallBase(), (by not calling base!).

For MethodTwo, we’ve issued a DoNotCallBase, and then a Returns statement.

Let’s add a new test to use this new, manual method:

[Fact]
public void Test2()
{
    // Arrange 
    MyFunkyClassTest myFunkyClassTest = new MyFunkyClassTest();
 
    // Act
    int result = myFunkyClassTest.MethodThree();
 
    // Assert
    Assert.Equal(2, result);
}

That’s much cleaner – why not always use manual mocks?

It is much cleaner if you always want MethodThree to return 5. Once you need it to return 2 then you have two choices, either you create a new mock class, or you start putting logic into your mock. The latter, if done wrongly can end up with code that is unreadable and difficult to maintain; and if done correctly will end up in a mini version of NSubstitute.

Finally, however well you write the mocks, as soon as you have more than one for a single class then every change to the class (for example, changing a method’s parameters or return type) results in a change to more than one test class.

It’s also worth mentioning again that this problem is one that has already been solved, cleanly, by dependency injection.

Short Walks – C# Pattern Matching to Match Ranges

Back in 2010, working at the time in a variety of languages, including VB, I asked this question on StackOverflow. In VB, you could put a range inside a switch statement, and I wanted to know how you could do that in C#. The (correct) answer at the time was that you can’t.

Fast forward just eight short years, and suddenly, it’s possible. The new feature of pattern matching in C# 7.0 has made this possible.

You can now write something like this (this is C# 7.1 because of Async Main):

static async Task Main(string[] args)
{            
    for (int i = 0; i <= 20; i++)
    {
        switch (i)
        {
            case var test when test <= 2:
                Console.WriteLine("Less than 2");
                break;
 
            case var test when test > 2 && test < 10:
                Console.WriteLine("Between 2 and 10");
                break;
 
            case var test when test >= 10:
                Console.WriteLine("10 or more");
                break;
        }
 
        await Task.Delay(500);
    }
 
    Console.ReadLine();
}

References

https://docs.microsoft.com/en-us/dotnet/csharp/pattern-matching

https://visualstudiomagazine.com/articles/2017/02/01/pattern-matching.aspx

Asynchronous Debugging

Everyone who has spent time debugging errors in code that has multiple threads knows the pain of pressing F10 and seeing the cursor jump to a completely different part of the system (that is, everyone who has ever tried to).

There are a few tools in VS2017 that make this process slightly easier; and this post attempts to provide a brief summary. Obviously the examples in this post are massively contrived.

Errors

Let’s start with an error occurring inside a parallel loop. Here’s some code that will cause the error:

static void Main(string[] args)
{
    Console.WriteLine("Hello World!");
 
    Parallel.For(1, 10, (i) => RunProcess(i));
 
    Console.ReadLine();
}
 
static void RunProcess(int i)
{
    Task.Delay(500).GetAwaiter().GetResult();
 
    Console.WriteLine($"Running {i}");
 
    if (i == 3) throw new Exception("error");
}

For some reason, I get an error when a few of these threads have started. I need a tool that tells me some details about the local variables in the threads specifically. Enter the Parallel Watch Window:

Launch Parallel Watch Window

Figure 1 – Launch Parallel Watch Window

This gives me a familiar interface, and tells me which thread I’m currently on:

Parallel Watch Window

Figure 2 – Parallel Watch Window

However, what I really want to see is the data local to the thread; what if I put “i” in the “Add Watch” cell:

Add a watch

Figure 3 – Add a watch

As you can see, I have a horizontal list of watch expressions, so I can monitor variables in multiple threads at a time.

Flagging a thread

We know there’s an issue with one of these threads, so one possibility is to flag that thread:

Flagging a thread

Figure 4 – Flagging a thread

Then you can select to show only flagged threads:

Filter flagged threads

Figure 5 – Filter flagged threads

Freezing non-relevant threads

The flags help to only trace the threads that you care about, but if you want to only run the threads that you care about, you can freeze the other threads:

Freeze Thread

Figure 6 – Freeze Thread

Once you’ve frozen a thread, a small pause icon appears, and that thread will stop:

Frozen Thread

Figure 7 – Frozen Thread

In order to freeze other threads, simply highlight all the relevant threads (Ctrl-A) and select Freeze.

It’s worth remembering that you can’t freeze a thread that doesn’t exist yet (so your breakpoint in a Parallel.For loop might only show half the threads).

Manual thread hopping

By using freeze, you can stop the debug message from jumping between threads. You can then manually control this process by simply selecting a thread and “Switch To Frame”:

Figure 8 – Switch to Frame

You can switch to a frozen frame, but as soon as you try to progress, you’ll flip back to the first non-frozen frame (unless you thaw it). The consequence of this is that, it is possible to switch to a frozen frame, freeze all other frames and then press F10 – you’re program will then stop dead.

Stack Trace

In a single threaded application (and in a multi-threaded application), you can always view the stack trace of a given line of executing code. There is also a Parallel Stack trace:

Parallel Stacks

Figure 9 – Parallel Stacks

Selecting any given method will give us the active threads, and allow switching:

Active Threads

Figure 10 – Active Threads

Parallel Stack Trace – Task View

The above view gives you a view of the created threads for your program; but most of the time, you won’t care what threads are created; only the tasks that you’ve spawned (they are not necessarily a 1 – 1 relationship. You can simply switch the view in this window to view Tasks instead:

Task View

Figure 11 – Task View

Tasks & Threads Windows

There is a tool that allows you to view all active, blocked and scheduled tasks:

Tasks Window

Figure 12 – Tasks Window

This allows you to freeze an entire task, switch to a given task, and Freeze All But This:

Freeze All But This

Figure 13 – Freeze All But This

There is an equivalent window for Threads. It is broadly the same idea; however, it does have one feature that the Tasks window does not, and that it the ability to rename a thread:

Rename a Thread

Figure 14 – Rename a Thread

Flags

The other killer feature both of these windows have is the flag feature. Simply flag a thread, switch to it, and then select “Show Only Flagged Threads” (little flag icon). If you now remove the breakpoints, you can step through only your thread or task!

Breakpoints

So, what to do where you have a breakpoint that you might only wish to fire for a single thread? Helpfully, the breakpoints window has a filter feature:

Filter breakpoints on thread Id

Figure 15 – Filter Breakpoints

References

https://msdn.microsoft.com/en-us/library/dd554943.aspx

https://stackoverflow.com/questions/5304752/how-to-debug-a-single-thread-in-visual-studio

Short Walks – Instantiating an Object Without calling the Constructor

One of the things that caught my attention at DDD North was the mention of a way to instantiate an object without calling its constructor.

Disclaimer

Typically, classes have code in their constructors that are necessary for their functionality, so you may find that doing this will cause your program to fall over.

System.Runtime.Serialization

The title of the namespace is probably the first thing that betrays the fact that you shouldn’t be doing this; but we’re already halfway down the rabbit hole!

Here’s some code that will create a class using reflection the normal way:

    static void Main(string[] args)
    {
        var test = Activator.CreateInstance<MyTestClass>();
        test.MyMethod();

        Console.WriteLine("Hello World!");
        Console.ReadLine();
    }

    public class MyTestClass
    {
        public MyTestClass()
        {
            Console.WriteLine("MyTestClass Initialise");
        }

        public string test1 { get; set; }

        public void MyMethod()
        {
            Console.WriteLine("Test MyMethod.");
        }
    }

The output is:

And here’s the code that circumvents the constructor:

        static void Main(string[] args)
        {
            var test2 = FormatterServices.GetUninitializedObject(typeof(MyTestClass)) as MyTestClass;
            test2.MyMethod();

            Console.WriteLine("Hello World!");
            Console.ReadLine();
        }

And we haven’t invoked the constructor:

Adding to an Existing Azure Blob

In this post I briefly cover the concept of Storage Accounts and Blob Storage; however, there are more to blobs than this simple use case. In this post, I’ll explore creating a blob file from a text stream, and then adding to that file.

As is stated in the post referenced above, Azure provides a facility for storing files in, what are known as, Azure Blobs.

In order to upload a file to a blob, you need a storage account, and a container. Setting these up is a relatively straightforward process and, again, is covered in the post above.

Our application here will take the form of a simple console app that will prompt the user for some text, and then add it to the file in Azure.

Set-up

Once you’ve set-up your console app, you’ll need the Azure NuGet Storage package.

Also, add the connection string to your storage account into the app.config:

<connectionStrings>
    <add name="Storage" connectionString="DefaultEndpointsProtocol=https;AccountName=testblob;AccountKey=wibble/dslkdsjdljdsoicj/rkDL7Ocs+aBuq3hpUnUQ==;EndpointSuffix=core.windows.net"/>
</connectionStrings>

Here’s the basic code for the console app:

static void Main(string[] args)
{
    Console.Write("Please enter text to add to the blob: ");
    string text = Console.ReadLine();
 
    UploadNewText(text);
 
    Console.WriteLine("Done");
    Console.ReadLine();
}

I’ll bet you’re glad I posted that, otherwise you’d have been totally lost. The following snippets are possible implementations of the method UploadNewText().

Uploading to BlockBlob

The following code will upload a file to a blob container:

string connection = ConfigurationManager.ConnectionStrings["Storage"].ConnectionString;
string fileName = "test.txt";
string containerString = "mycontainer";
 
using (MemoryStream stream = new MemoryStream())
using (StreamWriter sw = new StreamWriter(stream))
{
    sw.Write(text);
    sw.Flush();
    stream.Position = 0;
 
    CloudStorageAccount storage = CloudStorageAccount.Parse(connection);
    CloudBlobClient client = storage.CreateCloudBlobClient();
    CloudBlobContainer container = client.GetContainerReference(containerString);
    CloudBlockBlob blob = container.GetBlockBlobReference(fileName);
    blob.UploadFromStream(stream);
}

(note that the name of the container in this code is case sensitive)

If we have a look at the storage account, a text file has, indeed been created:

New Blob

But, what if we want to add to that? Well, running the same code again will work, but it will replace the existing file. To prove that, I’ve changed the text to “Test data 2” and run it again:

Test Data

So, how do we update the file? Given that we can update it, one possibility is to download the existing file, add to it and upload it again; that would look something like this:

string connection = ConfigurationManager.ConnectionStrings["Storage"].ConnectionString;
string fileName = "test.txt";
string containerString = "mycontainer";
 
CloudStorageAccount storage = CloudStorageAccount.Parse(connection);
CloudBlobClient client = storage.CreateCloudBlobClient();
CloudBlobContainer container = client.GetContainerReference(containerString);
CloudBlockBlob blob = container.GetBlockBlobReference(fileName);
 
using (MemoryStream stream = new MemoryStream())
{
    blob.DownloadToStream(stream);
 
    using (StreamWriter sw = new StreamWriter(stream))
    {
        sw.Write(text);
        sw.Flush();
        stream.Position = 0;
 
        blob.UploadFromStream(stream);
    }
}

This obviously means two round trips to the server, which isn’t the best thing in the world. Another possible option is to use the Append Blob…

Azure Append Blob Storage

There is a blob type that allows you to add to it without actually touching it; for example:

string connection = ConfigurationManager.ConnectionStrings["Storage"].ConnectionString;
string fileName = "testAppend.txt";
string containerString = "mycontainer";
 
CloudStorageAccount storage = CloudStorageAccount.Parse(connection);
CloudBlobClient client = storage.CreateCloudBlobClient();
CloudBlobContainer container = client.GetContainerReference(containerString);
CloudAppendBlob blob = container.GetAppendBlobReference(fileName);
if (!blob.Exists()) blob.CreateOrReplace();
 
using (MemoryStream stream = new MemoryStream())
using (StreamWriter sw = new StreamWriter(stream))
{
    sw.Write("Test data 4");
    sw.Flush();
    stream.Position = 0;
 
    blob.AppendFromStream(stream);                
}

There are a few things to note here:

  • The reason that I changed the name of the blob is that you can’t append to a BlockBlob (at least not using an AppendBlob); so it has to have been created for the purpose of appending.
  • While UploadFromStream will just create the file if it doesn’t exist, with the AppendBlob, you need to do it explicitly.

PutBlock

The final alternative here is to use PutBlock. This can bridge the gap, by allowing the addition of blocks into an existing block blob. However, you either need to maintain the Block ID list manually, or download the existing block list; here’s an example of creating, or adding to a file using the PutBlock method:

string connection = ConfigurationManager.ConnectionStrings["Storage"].ConnectionString;
string fileName = "test4.txt";
string containerString = "mycontainer";
 
CloudStorageAccount storage = CloudStorageAccount.Parse(connection);
CloudBlobClient client = storage.CreateCloudBlobClient();
CloudBlobContainer container = client.GetContainerReference(containerString);
CloudBlockBlob blob = container.GetBlockBlobReference(fileName);
 
ShowBlobBlockList(blob);
 
using (MemoryStream stream = new MemoryStream())
using (StreamWriter sw = new StreamWriter(stream))
{
    sw.Write(text);
    sw.Flush();
    stream.Position = 0;
 
    double seconds = (DateTime.Now - new DateTime(2000, 1, 1)).TotalSeconds;
    string blockId = Convert.ToBase64String(
        ASCIIEncoding.ASCII.GetBytes(seconds.ToString()));
 
    Console.WriteLine(blockId);
    //string blockHash = GetMD5HashFromStream(bytes);                
 
    List<string> newList = new List<string>();
    if (blob.Exists())
    {
        IEnumerable<ListBlockItem> blockList = blob.DownloadBlockList();
 
        newList.AddRange(blockList.Select(a => a.Name));
    }
 
    newList.Add(blockId);
 
    blob.PutBlock(blockId, stream, null);
    blob.PutBlockList(newList.ToArray());
}

The code above owes a lot to the advice given on this Stack Overflow question.

In order to avoid conflicts in the Block Ids, I’ve used a count of seconds since an arbitrary date. Obviously, this won’t work in all cases. Further, it’s worth noting that the code above still does two trips to the server (it has to download the block list).

The commented MD5 hash allows you to provide some form of check on the data being valid, should you choose to use it.

What is ShowBlobBlockList(blob)?

The following function will give some details relating to the existing blocks (it is shamelessly plagiarised from here):

public static void ShowBlobBlockList(CloudBlockBlob blockBlob)
{
    if (!blockBlob.Exists()) return;
 
    IEnumerable<ListBlockItem> blockList = blockBlob.DownloadBlockList(BlockListingFilter.All);
    int index = 0;
    foreach (ListBlockItem blockListItem in blockList)
    {
        index++;
        Console.WriteLine("Block# {0}, BlockID: {1}, Size: {2}, Committed: {3}",
            index, blockListItem.Name, blockListItem.Length, blockListItem.Committed);
    }
}

Summary

Despite being an established technology, these methods and techniques are sparsely documented on the web. Obviously, there are Microsoft docs, and they are helpful, but, unfortunately, not exhaustive.

References

https://stackoverflow.com/questions/33088964/append-to-azure-append-blob-using-appendtextasync-results-in-missing-data

https://docs.microsoft.com/en-us/rest/api/storageservices/understanding-block-blobs–append-blobs–and-page-blobs

http://www.c-sharpcorner.com/UploadFile/40e97e/windows-azure-blockblob-putblock-method/

https://docs.microsoft.com/is-is/rest/api/storageservices/put-block

https://www.red-gate.com/simple-talk/cloud/platform-as-a-service/azure-blob-storage-part-4-uploading-large-blobs/

https://stackoverflow.com/questions/46368954/can-putblock-be-used-to-append-to-an-existing-blockblob-in-azure