Why Do Lambda Expressions in Loops Produce Unexpected Results?

Lambda Expressions and Loop Iteration Variables: A Potential Pitfall

Using loop iteration variables directly within lambda expressions is generally discouraged. This practice can produce unexpected outcomes due to how lambda expressions handle variable capture.

Lambda expressions capture the reference to a variable, not its value at the time of capture. Therefore, when a loop iterates, the lambda expressions created within the loop all reference the same, ever-changing iteration variable. This leads to all lambdas ultimately using the variable's final value after the loop completes, not its value at the time each lambda was created.

Let's illustrate this with a VB.NET example:

Dim actions = new List(Of Action)()

For i As Integer = 0 To 9
    actions.Add(New Action(Function() Console.WriteLine(i)))
Next

For Each action As Action In actions
    action()
End For

You might expect this to print 0 through 9. Instead, it prints 10 ten times! This is because each Action in the actions list references the same i, and by the time the second loop executes, i has already reached its final value of 10.

The problem is amplified when the loop body has side effects impacting the variable referenced by the lambda expression. This can lead to difficult-to-debug inconsistencies.

The Solution: Create a Local Copy

To prevent this, create a local copy of the iteration variable inside the loop:

Dim actions = new List(Of Action)()

For i As Integer = 0 To 9
    Dim localI As Integer = i ' Create a local copy
    actions.Add(New Action(Function() Console.WriteLine(localI)))
Next

For Each action As Action In actions
    action()
End For

Now, each lambda expression captures a unique localI variable, holding its value at the time of creation. The output will correctly be 0 through 9. This simple change ensures each lambda operates with its own independent value, avoiding unexpected behavior.

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