Question

Apologies if this question has been asked already, but suppose we have this code (I’ve run it with Mono 2.10.2 and compiled with gmcs 2.10.2.0):

using System;

public class App {
    public static void Main(string[] args) {
        Func<string> f = null;
        var strs = new string[]{
            "foo",
            "bar",
            "zar"
        };

        foreach (var str in strs) {
            if ("foo".Equals(str)) 
                f = () => str;
        }
        Console.WriteLine(f());     // [1]: Prints 'zar'

        foreach (var str in strs) {
            var localStr = str;
            if ("foo".Equals(str))
                f = () => localStr;
        }
        Console.WriteLine(f());     // [2]: Prints 'foo'

        { int i = 0;
        for (string str; i < strs.Length; ++i) {
            str = strs[i];
            if ("foo".Equals(str)) 
                f = () => str;
        }}
        Console.WriteLine(f());     // [3]: Prints 'zar'
    }
}

It seems logical that [1] print the same as [3]. But to be honest, I somehow expected it to print the same as [2]. I somehow believed the implementation of [1] would be closer to [2].

Question: Could anyone please provide a reference to the specification where it tells exactly how the str variable (or perhaps even the iterator) is captured by the lambda in [1].

I guess what I am looking for is the exact implementation of the foreach loop.

Answer 1

You asked for a reference to the specification; the relevant location is section 8.8.4, which states that a “foreach” loop is equivalent to:

    V v;
    while (e.MoveNext()) {
        v = (V)(T)e.Current;
        embedded-statement
    }

Note that the value v is declared outside the while loop, and therefore there is a single loop variable. That is then closed over by the lambda.

UPDATE

Because so many people run into this problem the C# design and compiler team changed C# 5 to have these semantics:

    while (e.MoveNext()) {
        V v = (V)(T)e.Current;
        embedded-statement
    }

Which then has the expected behaviour — you close over a different variable every time. Technically that is a breaking change, but the number of people who depend on the weird behaviour you are experiencing is hopefully very small.

Be aware that C# 2, 3, and 4 are now incompatible with C# 5 in this regard. Also note that the change only applies to foreach, not to for loops.

See http://ericlippert.com/2009/11/12/closing-over-the-loop-variable-considered-harmful-part-one/ for details.

---

Commenter abergmeier states:

C# is the only language that has this strange behavior.

This statement is categorically false. Consider the following JavaScript:

var funcs = [];
var results = [];
for(prop in { a : 10, b : 20 })
{
  funcs.push(function() { return prop; });
  results.push(funcs[0]());
}

abergmeier, would you care to take a guess as to what are the contents of results?