• Getting started
• Creating a substitute
• Setting a return value
• Return for specific args
• Return for any args
• Return from a function
• Multiple return values
• Replacing return values
• Checking received calls
• Clearing received calls
• Argument matchers
• Callbacks, void calls and When..Do
• Throwing exceptions
• Safe configuration and overlapping calls
• Raising events
• Auto and recursive mocks
• Setting out and ref args
• Actions with argument matchers
• Checking call order
• Partial subs and test spies
• Return for all calls of a type
• Threading
• Compatibility argument matchers
• NSubstitute.Analyzers
• How NSubstitute works
• Search

Setting a return value

The following examples relate to substituting for the following interface:

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

For methods

To set a return value for a method call on a substitute, call the method as normal, then follow it with a call to NSubstitute’s Returns() extension method.

var calculator = Substitute.For<ICalculator>();
calculator.Add(1, 2).Returns(3);

This value will be returned every time this call is made. Returns() will only apply to this combination of arguments, so other calls to this method will return a default value instead.

//Make a call return 3:
calculator.Add(1, 2).Returns(3);
Assert.AreEqual(calculator.Add(1, 2), 3);
Assert.AreEqual(calculator.Add(1, 2), 3);

//Call with different arguments does not return 3
Assert.AreNotEqual(calculator.Add(3, 6), 3);

For properties

The return value for a property can be set in the same way as for a method, using Returns(). You can also just use plain old property setters for read/write properties; they’ll behave just the way you expect them to.

calculator.Mode.Returns("DEC");
Assert.AreEqual(calculator.Mode, "DEC");

calculator.Mode = "HEX";
Assert.AreEqual(calculator.Mode, "HEX");

More ways of setting return values

This covers the very basics of setting a return value, but NSubstitute can do much more. Read on for other approachs, including matching specific arguments, ignoring arguments, using functions to calculate return values and returning multiple results.