Question

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Why is it bad to use a iteration variable in a lambda expression

Why do I get: “iteration variable in a lambda expression may have unexpected results”? Suppose I have the following code:

  Dim writeAbleColumns As String() = {"IsSelected", "IsFeeExpense", "IsSubscriptionRedemption"}
  With grid
     For Each column As DataGridViewColumn In .Columns
      column.ReadOnly = Not Array.Exists(writeAbleColumns, Function(arrElement) column.Name = arrElement)
      Next
  End With

I get the warning:

Warning 1   Using the iteration variable in a lambda expression may have unexpected results.  Instead, create a local variable within the loop and assign it the value of the iteration variable.

I don’t understand why changing my code to the following changes anything:

  Dim writeAbleColumns As String() = {"IsSelected", "IsFeeExpense", "IsSubscriptionRedemption"}
  With grid
     For Each column As DataGridViewColumn In .Columns
      Dim c As DataGridViewColumn = column
      column.ReadOnly = Not Array.Exists(writeAbleColumns, Function(arrElement) c.Name = arrElement)
      Next
  End With

Fundamentally nothing changes except the warning disappears. I just have another variable point to my variable. Why the warning? What unexpected things might happen?

Answer 1

The lambda is bound to the variable, not to the value the variable had when the lambda was turned into a delegate. As the loop variable updates, every lambda created bound to that variable also sees the changed value of the variable, which you might not want. Creating a new variable every time you iterate the loop binds each lambda over a new, different, unchanging variable.

This is a major pain point in C# and VB. In C# 5 and VB 11 we are changing the loop closure semantics to mitigate this problem.

For more information see

Is there a reason for C#'s reuse of the variable in a foreach?

and the last few paragraphs of Tim’s article:

http://msdn.microsoft.com/en-us/magazine/cc163362.aspx

and my article:

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