Testing private methods with C# 4.0

Here's another practical use for dynamic in C# 4: testing private methods. I won't discuss here whether this should or shouldn't be done, that's been argued to death for years. Instead, I'll show how it can be done easier with C# 4, without resorting to reflection (at least not directly) or stubs, while also reducing some of the friction associated with doing TDD in statically-typed languages like C#.

The answer is very simple: we just walk away from static typing using C# dynamic:

public class Service {
    private int Step1() {
        return 1;
    }
}

[TestClass]
public class TransparentObjectTests {
    [TestMethod]
    public void PrivateMethod() {
        dynamic s = new Service().AsTransparentObject();
        Assert.AreEqual(1, s.Step1());
    }
}

When going dynamic, it's kind of like turning off the compiler, so, for example, it won't bother you at compile-time if a method isn't present. Instead, you'll just get an exception when you run your test, which is more like the dynamic language way of testing things.

Here's the code that enables this. Just like my last post, it's just a spike, it's incomplete and I didn't test it properly, but it could serve as a base for a more complete implementation:

using System;
using System.Dynamic;
using System.Linq;
using System.Reflection;
using Microsoft.CSharp.RuntimeBinder;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace DynamicReflection {
    public class Service {
        private int Step1() {
            return 1;
        }

        protected int Step2() {
            return 2;
        }

        internal int Step3() {
            return 3;
        }

        private int Prop { get; set; }

        public int Execute(int a) {
            return Step1() + Step2() + Step3() + a;
        }

        private string ExecuteGeneric<T>(T a) {
            return a.ToString();
        }
    }

    [TestClass]
    public class TransparentObjectTests {
        [TestMethod]
        public void PublicMethod() {
            dynamic s = new Service().AsTransparentObject();
            Assert.AreEqual(10, s.Execute(4));
        }

        [TestMethod]
        public void PrivateMethod() {
            dynamic s = new Service().AsTransparentObject();
            Assert.AreEqual(1, s.Step1());
        }

        [TestMethod]
        public void ProtectedMethod() {
            dynamic s = new Service().AsTransparentObject();
            Assert.AreEqual(2, s.Step2());
        }

        [TestMethod]
        public void InternalMethod() {
            dynamic s = new Service().AsTransparentObject();
            Assert.AreEqual(3, s.Step3());
        }

        [TestMethod]
        public void PrivateProperty() {
            dynamic s = new Service().AsTransparentObject();
            Assert.AreEqual(0, s.Prop);
        }

        [TestMethod]
        [ExpectedException(typeof(ArgumentException))]
        public void GenericPrivateMethod_type_mismatch() {
            dynamic s = new Service().AsTransparentObject();
            s.ExecuteGeneric<int>("lalal"); // type param mismatch
        }

        [TestMethod]
        public void GenericPrivateMethod() {
            dynamic s = new Service().AsTransparentObject();
            Assert.AreEqual("1", s.ExecuteGeneric<int>(1));
        }
    }

    public static class TransparentObjectExtensions {
        public static dynamic AsTransparentObject<T>(this T o) {
            return new TransparentObject<T>(o);
        }
    }

    public class TransparentObject<T> : DynamicObject {
        private readonly T target;

        public TransparentObject(T target) {
            this.target = target;
        }

        public override bool TryGetMember(GetMemberBinder binder, out object result) {
            var members = typeof(T).GetMember(binder.Name, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
            var member = members[0];
            if (member is PropertyInfo) {
                result = (member as PropertyInfo).GetValue(target, null);
                return true;
            }
            if (member is FieldInfo) {
                result = (member as FieldInfo).GetValue(target);
                return true;
            }
            result = null;
            return false;
        }

        public override bool TryInvokeMember(InvokeMemberBinder binder, object[] args, out object result) {
            var csBinder = binder as CSharpInvokeMemberBinder;
            var method = typeof(T).GetMethod(binder.Name, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
            if (method == null)
                throw new ApplicationException(string.Format("Method '{0}' not found for type '{1}'", binder.Name, typeof(T)));
            if (csBinder.TypeArguments.Count > 0)
                method = method.MakeGenericMethod(csBinder.TypeArguments.ToArray());
            result = method.Invoke(target, args);
            return true;
        }
    }
}

UPDATE 5/17/2010: Igor Ostrovsky recently got the same idea outlined here and has written a proper implementation of this.