An interesting change in C# 5.0 involves the handling of foreach iteration variables when used in closure scope. This is technically a breaking change, although it is really hard to think of a case where the old behavior was legitimately desired.

Consider (stolen from the spec doc):

  int[] values = { 7, 9, 13 };
  Action f = null;
  foreach (var value in values)
  {
      if (f == null) f = () => Console.WriteLine("First value: " + value);
  }
  f();

In previous versions of C#, there is a single value variable is in the scope of the entire loop, so the above code would actually print out the last value in the list because that was the value of value (sorry) when the loop finished. In C# 5.0, a value variable is actually in the scope of each individual iteration (so within the braces). So now if you use your iteration variable in a closure scope it won't change on you when the loop iterates and the above code actually prints out the first value.

The same cannot be said for a for loop. It still functions as it always has.