A colleague just came to me asking for advice on how to set the value of a referenced parameter with Moq; after nearly berating him for not checking Google I was surprised to find examples out there don't actually make it that obvious to the uninitiated.

For the purposes of this quick post I've created a dummy interface and associated method to mock, and then a quick method to test the mocked interface. This might seem long-winded but it's standard measure in our team to ensure it's firmly covered.

public interface ISomeThingToMock
{
    void MockWithRefParam(int someOtherParam, StringBuilder stringBuilder);
}

Let's show how a class might implement the interface:

public class SomeThingToMock : ISomeThingToMock
{
    public void MockWithRefParam(int someOtherParam, StringBuilder stringBuilder) {
        stringBuilder.Append("ThisWasNotMocked");
    }
}

Now I'm going to create a quick method that uses the above so our example is obvious:

public class MockTester
{
    public string TestThis(ISomeThingToMock myMock) {
        StringBuilder s = new StringBuilder();
        myMock.MockWithRefParam(10, s);
        return s.ToString();
    }
}

Using NUnit as our testing framework together with Moq this is a quick example test to prove our theory:

using System.Text;
using NUnit.Framework;
using Moq;
using ReferenceParameterMoqExample;

namespace ReferenceParameterMoqExampleTests
{
    [TestFixture]
    public class SomeThingToMockTests
    {
        private MockTester _MockTester;
        private Mock _SomeThingToMock;

        [Setup]
        public void TestSetup() {
            _MockTester = new MockTester();
            _SomeThingToMock = new Mock();
        }

        [Test]
        public void TestThis_WithRefParam_ReturnsSpecificValue() {
            _SomeThingToMock
                .Setup(x => x.MockWithRefParam(It.IsAny(), It.IsAny()))
                .Callback((int i, StringBuilder s) => s.Append("ThisWasMocked"));

            Assert.AreEqual("ThisWasMocked", _MockTester.TestThis(_SomeThingToMock.Object));
        }
    }
}