Capturing The Geek

Setting reference parameters in Moq

Tuesday 18 May 2010 at 12:18 pm. Used tags: moq, nunit, parameter, reference, return, test, value

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<ISomeThingToMock> _SomeThingToMock;

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

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

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

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