Avoid capturing the loop variable of foreach statements in lambda or delegate functions because the value captured varies depending on the version of C# used. In C# 5.0 and above the value captured is the original value but in earlier versions of C# the value captured is the current value. If the function is used outside of the loop, then the function will behave differently in different versions of C#.

Make a copy of the loop variable, which can be safely captured in the lambda function without being changed by the loop.

The following example creates a list of functions, where each function is intended to print out a different number. However on older compilers, the actual output is 10 10 10 10 10 10 10 10 10 10.

The bug can be fixed by introducing a new variable (i2) which does not capture the loop variable.

The output of the new code is 1 2 3 4 5 6 7 8 9 10 as intended.

Eric Lippert's Blog: Closing over the loop variable considered harmful.