Short Walks – NSubstitute – Subclassing and Partial Substitutions

I’ve had this issue a few times recently. Each time I have it, after I’ve worked out what it was, it makes sense, but I keep running into it. The resulting frustration is this post – that way, it’ll come up the next time I search for it on t’internet.

The Error

The error is as follows:

“NSubstitute.Exceptions.CouldNotSetReturnDueToNoLastCallException: ‘Could not find a call to return from.”

Typically, it seems to occur in one of two circumstances: substituting a concrete class and partially substituting a concrete class; that is:

var myMock = Substitute.ForPartsOf<MyClass>();


var myMock = Substitute.For<MyClass>();


If you were to manually mock out an interface, how would you do that? Well, say you had IMyClass, you’d just do something like this:

public class MyClassMock : IMyClass 
	// New and imaginative behaviour goes here

All’s good – you get a brand new implementation of the interface, and you can do anything with it. But what would you do if you were trying to unit test a method inside MyClass that called another method inside MyClass; for example:

public class MyClass : IMyClass
	public bool Method1()
		int rowCount = ReadNumberOfRowsInFileOnDisk();
		Return rowCount > 10;
	public int ReadNumberOfRowsInFileOnDisk()
		// Opens a file, reads it, and returns the number of rows

(Let’s not get bogged down in how realistic this scenario is, or whether or not this is good practice – it illustrates a point)

If you want to unit test Method1(), but don’t want to actually read a file from the disk, you’ll want to replace ReadNumberOfRowsInFileOnDisk(). The only real way that you can do this is to subclass the class; for example:

public class MyClassMock : MyClass

You can now test the behaviour on MyClass, via MyClassMock; however, you still can’t* override the method ReadNumberOfRowsInFileOnDisk() because it isn’t virtual. If you make the method virtual, you can override it in the subclass.

The same is true with NSubstitute – if you want to partially mock a class in this way, it follows the same rules as you must if you would to roll your own.


* There may, or may not be one or two ways to get around this restriction, but let’s at least agree that they are, at best, unpleasant.

This site uses Akismet to reduce spam. Learn how your comment data is processed.