Can anyone explain to me why using iteration variable in a lambda expression is a bad idea

Question

http://msdn.microsoft.com/en-us/library/bb763133.aspx

Module Module1
    Sub Main()
        Dim array1 As Func(Of Integer)() = New Func(Of Integer)(4) {}

        For i As Integer = 0 To 4
            array1(i) = Function() i
        Next

        For Each funcElement In array1
            System.Console.WriteLine(funcElement())
        Next

    End Sub
End Module

It says the result will always be 5 namely the final value of i. How come?

They don't put the iteration variable in the "closure"?

possible duplicate of Why is it bad to use a iteration variable in a lambda expression — MarkJ
– MarkJ, Commented Feb 16, 2012 at 7:55

Harsh · Accepted Answer · 2012-02-16 07:35:19Z

4

The problem occurs because lambda expressions do not execute when they are constructed but rather when they are invoked.

See the link below: http://blogs.msdn.com/b/vbteam/archive/2007/07/26/closures-in-vb-part-5-looping.aspx

Hope it helps.

answered Feb 16, 2012 at 7:35

Harsh

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3 Comments

user4951 Over a year ago

you mean the closure of the lambda expression all contain the same pointer to the same variable i? Is that how it works?

Eric Lippert Over a year ago

@JimThio: Correct. Lambdas close over variables, not over values. Note that we are changing this behaviour slightly in C# 5. I do not recall if a similar change is being made in VB.

user4951 Over a year ago

Wow you're from microsoft? Tell Bill Gates I say hi. I am a fan.

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