Getting started

Adding NSubstitute to your test project

First add the NSubstitute NuGet package to your test project using NuGet (either the command line executable, or via the package manager in your IDE).

> Install-Package NSubstitute

It is optional but recommended to also install NSubstitute.Analyzers.CSharp for C# projects, or NSubstitute.Analyzers.VisualBasic for VB projects. NSubstitute will work without the analysers installed, but these packages will help detect potential misuses of the NSubstitute API.

> Install-Package NSubstitute.Analyzers.CSharp
// or
> Install-Package NSubstitute.Analyzers.VisualBasic

Using NSubstitute in a test fixture

So now you are staring at a blank test fixture (created with your favourite unit testing framework; for these examples we’re using NUnit), and are wondering where to start.

First, add using NSubstitute; to your current C# file. This will give you everything you need to start substituting.

Now let’s say we have a basic calculator interface:

public interface ICalculator
    int Add(int a, int b);
    string Mode { get; set; }
    event EventHandler PoweringUp;

We can ask NSubstitute to create a substitute instance for this type. We could ask for a stub, mock, fake, spy, test double etc., but why bother when we just want to substitute an instance we have some control over?

calculator = Substitute.For<ICalculator>();

⚠️ Note: NSubstitute will only work properly with interfaces, or with class members that are overridable from the test assembly. Be very careful substituting for classes with non-virtual or internal virtual members, as real code could be inadvertently executed in your test. See Creating a substitute and How NSubstitute works for more information. Also make sure to install NSubstitute.Analyzers which can warn about many of these cases (but not all of them; be careful with classes!).

Now we can tell our substitute to return a value for a call:

calculator.Add(1, 2).Returns(3);
Assert.That(calculator.Add(1, 2), Is.EqualTo(3));

We can check that our substitute received a call, and did not receive others:

calculator.Add(1, 2);
calculator.Received().Add(1, 2);
calculator.DidNotReceive().Add(5, 7);

If our Received() assertion fails, NSubstitute tries to give us some help as to what the problem might be:

NSubstitute.Exceptions.ReceivedCallsException : Expected to receive a call matching:
    Add(1, 2)
Actually received no matching calls.
Received 2 non-matching calls (non-matching arguments indicated with '*' characters):
    Add(*4*, *7*)
    Add(1, *5*)

We can also work with properties using the Returns syntax we use for methods, or just stick with plain old property setters (for read/write properties):

Assert.That(calculator.Mode, Is.EqualTo("DEC"));

calculator.Mode = "HEX";
Assert.That(calculator.Mode, Is.EqualTo("HEX"));

NSubstitute supports argument matching for setting return values and asserting a call was received:

calculator.Add(10, -5);
calculator.Received().Add(10, Arg.Any<int>());
calculator.Received().Add(10, Arg.Is<int>(x => x < 0));

We can use argument matching as well as passing a function to Returns() to get some more behaviour out of our substitute (possibly too much, but that’s your call):

   .Add(Arg.Any<int>(), Arg.Any<int>())
   .Returns(x => (int)x[0] + (int)x[1]);
Assert.That(calculator.Add(5, 10), Is.EqualTo(15));

Returns() can also be called with multiple arguments to set up a sequence of return values.

calculator.Mode.Returns("HEX", "DEC", "BIN");
Assert.That(calculator.Mode, Is.EqualTo("HEX"));
Assert.That(calculator.Mode, Is.EqualTo("DEC"));
Assert.That(calculator.Mode, Is.EqualTo("BIN"));

Finally, we can raise events on our substitutes (unfortunately C# dramatically restricts the extent to which this syntax can be cleaned up):

bool eventWasRaised = false;
calculator.PoweringUp += (sender, args) => eventWasRaised = true;

calculator.PoweringUp += Raise.Event();

That’s pretty much all you need to get started with NSubstitute. Read on for more detailed feature descriptions, as well as for some of the less common requirements that NSubstitute supports.