Can Moq Assign Values to Out Parameters?

Output parameter assignment in Moq

Question:

Can Moq be used to assign values ​​to output parameters?

Answer:

Yes, you can use Moq 3.0 and above to assign values ​​to output parameters. Here’s how:

For output parameters:

public interface IService
{
    void DoSomething(out string a);
}

[TestMethod]
public void Test()
{
    var service = new Mock<IService>();
    string expectedValue = "value";
    service.Setup(s => s.DoSomething(out expectedValue));

    string actualValue;
    service.Object.DoSomething(out actualValue);
    Assert.AreEqual(expectedValue, actualValue);
}

Moq records the value assigned to an output parameter during setup and returns that value during testing.

For reference parameters:

Moq does not currently support this feature. However, you can use Rhino Mocks, or use a constrained Action as a workaround:

Use constrained Action:

public interface IService
{
    void DoSomething(ref string a);
}

[TestMethod]
public void Test()
{
    var service = new Mock<IService>();
    string value = "initial";

    service.Setup(s => s.DoSomething(ref It.Ref<string>.IsAny))
        .Callback<string>(s => s = "expectedValue");

    service.Object.DoSomething(ref value);
    Assert.AreEqual("expectedValue", value);
}

Here we use the It.Ref<t>.IsAny</t> constraint to match any input reference parameter and set its value in the callback.

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