PatternMatchExtensions.cs

using System;

/// <summary>
/// Extension class for pattern matching workaround in older version of C#
/// </summary>
public static class PatternMatchExtension
{
    public interface IMatcher<T>
        where T : class
    {
        /// <summary>
        /// Invokes Action when pattern object is of type TV
        /// </summary>
        /// <typeparam name="TV">subtype of T</typeparam>
        /// <param name="Action">Action to invoke</param>
        /// <returns></returns>
        IMatcher<T> Case<TV>(Action<TV> Action)
            where TV : T;

        /// <summary>
        /// Invokes Action when other matches hasn't succeed
        /// </summary>
        /// <param name="Action">Action to invoke</param>
        void Default(Action<T> Action);
    }

    /// <summary>
    /// Starts the chain of matching cases. Call Case and Default methods of returned object.
    /// </summary>
    /// <typeparam name="T">Object type</typeparam>
    /// <param name="Object">Object for pattern matching</param>
    /// <returns>Pattern matching monad <see cref="IMatcher{T}"/></returns>
    public static IMatcher<T> Match<T>(this T Object)
        where T : class
    {
        return new Matcher<T>(Object);
    }

    private class Matcher<T> : IMatcher<T>
        where T : class
    {
        public Matcher(T Object)
        {
            this.obj = Object;
        }

        public IMatcher<T> Case<TV>(Action<TV> Action)
            where TV : T
        {
            if (obj is TV)
            {
                Action((TV)obj);
                return new Bypass<T>();
            }
            return this;
        }

        public void Default(Action<T> Action)
        {
            Action(obj);
        }

        private T obj;
    }

    private class Bypass<T> : IMatcher<T>
        where T : class
    {
        public IMatcher<T> Case<TV>(Action<TV> Action)
            where TV : T
        {
            return this;
        }

        public void Default(Action<T> Action)
        {
        }
    }
}