C# - closures over class fields inside an initializer?

Posted by Richard Berg on Stack Overflow See other posts from Stack Overflow or by Richard Berg
Published on 2010-03-15T23:59:57Z Indexed on 2010/03/16 0:09 UTC
Read the original article Hit count: 544

Consider the following code:

using System;

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            var square = new Square(4);
            Console.WriteLine(square.Calculate());
        }
    }

    class MathOp
    {        
        protected MathOp(Func<int> calc) { _calc = calc; }
        public int Calculate() { return _calc(); }
        private Func<int> _calc;
    }

    class Square : MathOp
    {
        public Square(int operand)
            : base(() => _operand * _operand)  // runtime exception
        {
            _operand = operand;
        }

        private int _operand;
    }
}

(ignore the class design; I'm not actually writing a calculator! this code merely represents a minimal repro for a much bigger problem that took awhile to narrow down)

I would expect it to either:

  • print "16", OR
  • throw a compile time error if closing over a member field is not allowed in this scenario

Instead I get a nonsensical exception thrown at the indicated line. On the 3.0 CLR it's a NullReferenceException; on the Silverlight CLR it's the infamous Operation could destabilize the runtime.

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