For-Loop and LINQ's deferred execution don't play well together

Question

The title suggests that i\'ve already an idea what\'s going on, but i cannot explain it. I\'ve tried to order a List dynamically by each \"colum

Answer 1

It's the same problem as lots of people had with foreach loops pre C# 5.

orderedValues = orderedValues.ThenBy(t => t[i]);

The value of i will not be evaluated until you call .ToList() at which point it is 2 since that's the exit condition of the for loop.

You can introduce a new local variable inside the for-loop to fix it:

for (int i = 1; i < minDim; i++)
{
    var tmp = i;
    orderedValues = orderedValues.ThenBy(t => t[tmp]);
}

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For more information you could take a look at Eric Lippert's blog post about Closing over the loop variable considered harmful.

Answer 2

This is probably happening because the value of i is not closed within the loop - when the loop exits, i will have a value of 2 and then t[i] will be evaluated because of deferred execution.

One solution is to create a closing variable within the loop:

int minDim = someValues.Min(t => t.GetLength(0));  // 2
IOrderedEnumerable<string[]> orderedValues = someValues.OrderBy(t => t[0]);
for (int i = 1; i < minDim; i++)
{
    var x = i;
    orderedValues = orderedValues.ThenBy(t => t[x]);
}
someValues = orderedValues.ToList();