Tag Archives: Activator.CreateInstance

Casting a C# Object From its Parent

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

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.

Generic Method to Clear a Class and Intelli-Test

Recently, I published this article on copying a class dynamically. I then found that I could use the same approach to clear a class. Here’s the method:

        private static void ClearClass<T>(T classToClear)
            if (classToClear == null)
                throw new Exception("Must not specify null parameters");

            var properties = classToClear.GetType().GetProperties();

            foreach (var p in properties.Where(prop => prop.CanWrite))
                p.SetValue(classToClear, GetDefault(p.PropertyType));

        /// <summary>
        /// Taken from http://stackoverflow.com/questions/474841/dynamically-getting-default-of-a-parameter-type
        /// </summary>
        /// <param name="type"></param>
        /// <returns></returns>
        public static object GetDefault(Type type)
            return type.IsValueType ? Activator.CreateInstance(type) : null;

As you can see, I had a little help from Jon Skeet with this one. Once I’d written it, I thought I’d have a play with the IntelliTest feature: if you right click the method and select “Create IntelliTest”, you’re presented with this:



It generated this:

    /// <summary>This class contains parameterized unit tests for Program</summary>
    [PexAllowedExceptionFromTypeUnderTest(typeof(ArgumentException), AcceptExceptionSubtypes = true)]
    public partial class ProgramTest
        /// <summary>Test stub for ClearClass(!!0)</summary>
        internal void ClearClassTest<T>(T classToClear)
            // TODO: add assertions to method ProgramTest.ClearClassTest(!!0)

The interesting thing about this, is that it can’t be found as a test. What actually happens is this creates an intelli-test, which, as far as I can see, you have to right-click on the created test and select “Run Intelli-test”. This then creates you an actual unit test:


It looks something like this:

namespace ConsoleApplication13.Tests
    public partial class ProgramTest

public void ClearClassTest861()

That then can be found and run:


Obviously, looking at the unit test, it’s not a particularly good one; it effectively tests that your code doesn’t crash, so it increases your code coverage, but doesn’t really test anything per-se.