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Returns

Updated on Mar 25, 2026

Use Returns to specify the value a mocked method or property returns when called. Returns applies only to non-void members - for void methods, use DoNothing or DoInstead instead.

The examples use the following interface:

C#
public interface IFoo
{
    int Bar { get; set; }
    int Echo(int myInt);
    int Execute(int myInt1, int myInt2);
}
VB
Public Interface IFoo
    Function Execute(ByVal int1 As Integer, ByVal int2 As Integer)
    Property Bar As Integer
    Function Echo(ByVal int As Integer) As Integer
End Interface

You may additionally want to check theCreate Mocks By Example article.

Assert Property Get Call

With Returns method you can change the return value from a property get call.

C#
[TestMethod]
public void ShouldAssertPropertyGetCall()
{
    // Arrange
    var foo = Mock.Create<IFoo>();

    Mock.Arrange(() => foo.Bar).Returns(10);


    // Act
    var actual = 0;
    actual = foo.Bar;

    // Assert
    Assert.AreEqual(10, actual);
}
VB
<TestMethod()>
Public Sub ShouldAssertPropertyGetCall()
    ' Arrange
    Dim foo = Mock.Create(Of IFoo)()

    Mock.Arrange(Function() foo.Bar).Returns(10)

    ' Act
    Dim actual = 0
    actual = foo.Bar

    ' Assert
    Assert.AreEqual(10, actual)
End Sub

In this example we arrange the Bar property get to return 10 when called. By acting with actual = foo.Bar; we assign 10 to actual, as foo.Bar will result in 10. Finally, a verification is asserted.

Assert Method Call with Matcher

A common case is to mock a method call to return a custom value in conjunction with a matcher.

C#
[TestMethod]
public void ShouldAssertMethodCallWithMatcher1()
{
    // Arrange
    var foo = Mock.Create<IFoo>();

    Mock.Arrange(() => foo.Echo(Arg.IsAny<int>())).Returns((int i) => ++i);

    // Act
    var actual = foo.Echo(10); 

    // Assert
    Assert.AreEqual(11, actual);
}
VB
<TestMethod()>
Public Sub ShouldAssertMethodCallWithMatcher1()
    ' Arrange
    Dim foo = Mock.Create(Of IFoo)()

    Mock.Arrange(Function() foo.Echo(Arg.IsAny(Of Integer)())).Returns(Function(i As Integer) System.Threading.Interlocked.Increment(i))

    ' Act
    Dim actual = foo.Echo(10)

    ' Assert
    Assert.AreEqual(11, actual)
End Sub

Here we use an Arg.IsAny matcher for the call to match calls to foo.Echo with any int argument. In the Returns method, instead of using a simple int value, we use a function to return the passed value incremented with 1.

You may also use a more complicated matcher. For example, you can make arrangement for passing exactly 10 to Echo method. With the following line of code we return exactly what has been passed:

C#
[TestMethod]
public void ShouldAssertMethodCallWithMatcher2()
{
  // Arrange
  var foo = Mock.Create<IFoo>();

  Mock.Arrange(() => foo.Echo(Arg.Matches<int>(x => x == 10))).Returns((int i) => i);

  // Act
  var actual = foo.Echo(10);

  // Assert
  Assert.AreEqual(10, actual);
}
VB
<TestMethod()>
Public Sub ShouldAssertMethodCallWithMatcher2()
    ' Arrange
    Dim foo = Mock.Create(Of IFoo)()

    Mock.Arrange(Function() foo.Echo(Arg.Matches(Of Integer)(Function(x) x = 10))).Returns(Function(i As Integer) i)

    ' Act
    Dim actual As Integer = foo.Echo(10)

    ' Assert
    Assert.AreEqual(10, actual)
End Sub

The following example mocks the Execute method so that it returns its second argument. We use lambda expression in the Returns body to select the desired argument. After we have arranged we act by calling foo.Execute(100, 10) and verify that it has returned actually what we expect, namely 10. Additionally, we verify that a call with any int as first argument and exactly 10 as second has been made.

C#
[TestMethod]
public void ShouldReturnWhateverSecondArgIs()
{
    // Arrange
    var foo = Mock.Create<IFoo>();

    Mock.Arrange(() => foo.Execute(Arg.IsAny<int>(), Arg.IsAny<int>())).Returns((int id, int i) => i);

    // Act, Assert
    Assert.AreEqual(foo.Execute(100, 10), 10);

    Mock.Assert(() => foo.Execute(Arg.IsAny<int>(), Arg.Matches<int>(x => x == 10)));
}
VB
<TestMethod()>
Public Sub ShouldReturnWhateverSecondArgIs()
    ' Arrange
    Dim foo = Mock.Create(Of IFoo)()

    Mock.Arrange(Function() foo.Execute(Arg.AnyInt, Arg.AnyInt)).Returns(Function(id As Integer, i As Integer) i)

    ' Act, Assert
    Assert.AreEqual(foo.Execute(100, 10), 10)

    Mock.Assert(Sub() foo.Execute(Arg.AnyInt, Arg.Matches(Of Integer)(Function(x) x = 10)))
End Sub

Refer to the Matchers help topic for more information about using matchers.

Executing Mocked Method In Same Test Method

Assume we have the following class:

C#
public class Foo
{
    public int Echo(int myInt)
    {
        throw new NotImplementedException();
    }
}
VB
Public Class Foo
    Public Function Echo(num As Integer) As Integer
        Throw New NotImplementedException
    End Function
End Class

JustMock supports executing mocked method in same test method without necessarily passing the type as an argument.

C#
[TestMethod]
public void ShouldExecuteMockForSameInstanceInSameContext()
{
    // Arrange
    var foo = Mock.Create<Foo>();

    Mock.Arrange(() => foo.Echo(Arg.AnyInt)).IgnoreInstance().Returns((int arg1) => arg1);

    // Assert
    Assert.AreEqual(new Foo().Echo(10), 10);
}
VB
<TestMethod()>
Public Sub ShouldExecuteMockForSameInstanceInSameContext()
    ' Arrange
    Dim foo = Mock.Create(Of Foo)()

    Mock.Arrange(Function() foo.Echo(Arg.AnyInt)).IgnoreInstance().Returns(Function(arg1 As Integer) arg1)

    ' Assert
    Assert.AreEqual(New Foo().Echo(10), 10)
End Sub

In this example, Echo is called on a new Foo instance, instead of a mocked one. However, the arrangement is applied.

Returns

Assert Property Get Call

Assert Method Call with Matcher

Executing Mocked Method In Same Test Method

See Also