DowncastToConcreteType.cs

// Suggested rule: warn on casts from interfaces to concrete types

using System;

namespace VS2013ConsoleApplication1.RoslynRule_samples
{
    public interface IMyInterface
    {
        void DoStuff();
    }

    public class MyClass1 : IMyInterface
    {
        public int Data { get { return new Random().Next(); } }

        public void DoStuff()
        {
            // TODO...
        }
    }

    public static class DowncastExampleProgram
    {
        static void EntryPoint()
        {
            IMyInterface interfaceRef = FindInterfaceInstanceFromSomewhere();

            // Worst-case: hard cast to a concrete type.
            // Hard-codes the assumption that there is only one implementation
            // of the interface -> fragile; causes maintenance issue.
            // It also suggests that the IMyInterface abstraction is
            // incomplete or incorrect: the caller is having to downcast to get
            // data because it isn't on the interface.
            MyClass1 class1 = (MyClass1)interfaceRef;
            int privateData = class1.Data;

            // 2. Slightly less bad version: at least it won't fail at runtime
            class1 = interfaceRef as MyClass1;
            if (class1 != null)
            {
                // ...
            }

            // Notes: you might validly want to downcast to a concrete
            // type in a unit test. Also, casting to another interface is generally
            // ok as a way of checking whether a behaviour is supported.

        }

        private static IMyInterface FindInterfaceInstanceFromSomewhere()
        {
            return new MyClass1();
        }
    }

}