IfThenElseExprs.cs

class Program
{
    static void Main()
    {
        var result =
            Expr.If(DateTime.Now.Second % 2 == 0)
            .Then(() =>
            {
                Console.WriteLine("Then clause handled.");
                return 0;
            })
            .Else(() =>
            {
                Console.WriteLine("Else clause handled.");
                return 1;
            });
        Console.WriteLine("Result: " + result.Value);
    }
}

class Expr
{
    public static ExprIf If(bool cond)
    {
        return new ExprIf { Condition = cond };
    }
}

class ExprIf
{
	public bool Condition { get; set;}
}

class ExprThen<T>
{
	public T Value { get; set; }
	public bool Handled { get; set;}
}

class ExprElse<T>
{
	public T Value { get; set;}
}

static class Extensions
{
	public static ExprThen<T> Then<T>(this ExprIf expr, Func<T> body)
    {
        if (expr.Condition)
        {
            return new ExprThen<T> { Value = body(), Handled = true };
        }
        return new ExprThen<T> { Value = default(T), Handled = false };
    }

    public static ExprElse<T> Else<T>(this ExprThen<T> expr, Func<T> body)
    {
		if (expr.Handled)
			return new ExprElse<T> { Value = expr.Value };
		return new ExprElse<T> { Value = body() };
	}
}