Implementing pattern matching in C#

Question

In Scala, you can use pattern matching to produce a result depending on the type of the input. For instance:

val title = content match {
    case blogPost: BlogPost => blogPost.blog.title + ": " + blogPost.title
    case blog: Blog => blog.title
}

In C#, I'd ideally like to be able to write:

var title = Visit(content,
    (BlogPost blogPost) => blogPost.Blog.Title + ": " + blogPost.Title,
    (Blog blog) => blog.Title
);

Is this possible? When I've tried writing it as a single method, I don't know how to specify the generics. The following implementation seems right, apart from getting the type checker to allow functions that accept subtypes of T:

    public TResult Visit<T, TResult>(T value, params Func<T, TResult>[] visitors)
    {
        foreach (var visitor in visitors)
        {
            if (visitor.Method.GetGenericArguments()[0].IsAssignableFrom(value.GetType()))
            {
                return visitor(value);
            }
        }
        throw new ApplicationException("No match");
    }

The closest I've gotten is to add the functions to an object individually, and then call visit on a value:

    public class Visitor<T, TResult>
    {
        private class Result
        {
            public bool HasResult;
            public TResult ResultValue;
        }

        private readonly IList<Func<T, Result>> m_Visitors = new List<Func<T, Result>>();

        public TResult Visit(T value)
        {
            foreach (var visitor in m_Visitors)
            {
                var result = visitor(value);
                if (result.HasResult)
                {
                    return result.ResultValue;
                }
            }
            throw new ApplicationException("No match");
        }

        public Visitor<T, TResult> Add<TIn>(Func<TIn, TResult> visitor) where TIn : T
        {
            m_Visitors.Add(value =>
            {
                if (value is TIn)
                {
                    return new Result { HasResult = true, ResultValue = visitor((TIn)value) };
                }
                return new Result { HasResult = false };
            });
            return this;
        }
    }

This can be used like so:

var title = new Visitor<IContent, string>()
    .Add((BlogPost blogPost) => blogPost.Blog.Title + ": " + blogPost.Title)
    .Add((Blog blog) => blog.Title)
    .Visit(content);

Any idea how to do this with a single method call?

Answer 1

Pattern matching is one of those lovely features mostly found in functional programming languages like F#. There is a great project going on in codeplex named Functional C#. Consider the following F# code:

let operator x = match x with
                 | ExpressionType.Add -> "+"

let rec toString exp = match exp with
                       | LambdaExpression(args, body) -> toString(body)
                       | ParameterExpression(name) -> name
                       | BinaryExpression(op,l,r) -> sprintf "%s %s %s" (toString l) (operator op) (toString r)

Using the Functional C# library, the C# equivalent would be:

var Op = new Dictionary<ExpressionType, string> { { ExpressionType.Add, "+" } };

Expression<Func<int,int,int>> add = (x,y) => x + y;

Func<Expression, string> toString = null;
 toString = exp =>
 exp.Match()
    .With<LambdaExpression>(l => toString(l.Body))
    .With<ParameterExpression>(p => p.Name)
    .With<BinaryExpression>(b => String.Format("{0} {1} {2}", toString(b.Left), Op[b.NodeType], toString(b.Right)))
    .Return<string>();