C# - how to test extension methods?

Question

I have created this C# extension method for a Person class:

public static class PersonExtensions {
    public static void Rename(this Person person, String newName) {
        person.Name = newName;
    }
}

How would I unit test this method? I have tried it, but the Rename method is not available from the PersonAccessor object.

Error was "the private-accessor for Rename was not found"

When I try PersonExtensions_Accessor.Rename(somePerson, newName), it says "there are some invalid arguments"

Answer 1

Extension methods are just syntactic sugar for a different way to reference the static method. Just call PersonExtensions.Rename(...) in your unit test.

Answer 2

I think that a good approach could be testing directly the extension method in an instance of Person.

Taking into account the method you have implemented, a sample code would be something like this:

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using PersonExtension; // Don't forget about reference it

namespace UnitTest {
    [TestClass]
    public class UnitTest {
        Person person;
        [TestInitialize]
        public void Init() {
            person = new Person("Person name");
        }

        [TestMethod]
        public void TestRename() {
            Assert.AreEqual("Person name", person.Name);
            person.Rename("New name");
            Assert.AreEqual("New name", person.Name);
        }
    }
}

Remember to reference both Person and PersonExtension classes and to have the correct hidden levels in the utility class in order to can access its methods