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 was 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.

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.