Anonymous C# delegate within a loop

Question

Hi all I am trying to write and anonymous delegate. as the integer variable is shared among the delegate I need it to be the local instance of every delegate such that rs[0] always gets nics[0], rs[1] always gets nics[1] and so on... how will I achieve this.

for (int i = 0; i < nics.Count; i++)
{
    rs[i] = new RollingSeries(monitor, new RollingSeries.NextValueDelegate(delegate()
    {
        return GetNetworkUtilization(nics[i]);
    }));
}

Be *extremely* careful about what you ask for. Do you want your first closure to always get the value in nics[0] *at the time the delegate is invoked*, or do you want the closure to get the value that happened to be in nics[0] *at the time the delegate was created*? Those are two different things; nics[0] is a variable, so its value can change over time. Do you want the closure to see that change, or ignore that change? — Eric Lippert, Nov 18 '09 at 16:34

score 23 · Accepted Answer · answered Nov 18 '09 at 16:14

23

Make a local copy of i:

   for (int i = 0; i < nics.Count; i++)
   {
         int j = i;
         rs[i] = new RollingSeries(monitor, new RollingSeries.NextValueDelegate(delegate()
         {
            return GetNetworkUtilization(nics[j]);
          }));
    }

The Beauty of Closures

answered Nov 18 '09 at 16:14

bruno conde

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Or an local copy of the item in the array. – Dykam Nov 18 '09 at 16:24

score 6 · Answer 2 · answered Nov 18 '09 at 16:14

Use a local to get a different value per iteration

for (int i = 0; i < nics.Count; i++)
   {
         int localI = i;
         rs[i] = new RollingSeries(monitor, new RollingSeries.NextValueDelegate(delegate()
         {
            return GetNetworkUtilization(nics[localI]);
          }));
    }

Heinzi · Answer 3 · 2009-11-18T16:24:39.063

4

Put int j = i inside your loop and refer to j within the lambda expression.

If you are curious about why this happens, here is an MSDN blog entry containing a detailed technical explanation: Closing over the loop variable considered harmful

edited Nov 18 '09 at 16:24

answered Nov 18 '09 at 16:14

Heinzi

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Anonymous C# delegate within a loop

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