#1,183 – How to Correctly Capture a for Loop Variable in a Lambda Expression

If you directly capture a for loop variable within a lambda expression, the value used when executing the expression will typically be the value of the variable at the time that the loop exits.  If you instead want to capture a variable whose value is the value of the for loop variable while the loop is executing, you can use a local variable.

            Action[] dels = new Action[3];

            // How to correctly capture for loop variable
            for (int i = 0; i < 3; i++)
            {
                int iLocal = i;
                dels[i] = () => Console.WriteLine(iLocal + 1);
            }

            // Prints 1/2/3
            foreach (Action d in dels)
                d();

1183-001

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About Sean
Software developer in the Twin Cities area, passionate about software development and sailing.

2 Responses to #1,183 – How to Correctly Capture a for Loop Variable in a Lambda Expression

  1. Claudio Rodrigues says:

    Why? What´s the difference between “int i” and int iLocal in this loop?

    • Sean says:

      iLocal is a variable local to the body of the loop. When it is captured by a lambda expression, each delegate instance captures a different variable because of the scope. This results in the (correct) 1/2/3 output. int i is a loop variable and there is only one of them. If you capture i instead of iLocal, you’ll get different results because each delegate instance will use the same copy of i. (See post #1,182).

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