Why aren't unassigned local variables automatically initialized?

Question

It seems like there is no way to have unassigned local variables in your code or check for them, as the compiler spits out Use of unassigned local variable erro

Answer 1

Consider the following code:

void blah(IDictionary dict)
{
  for (int i=0; i<10; i++)
  {
    if ((i & 11) != 0)
    {
        int j;
        dict.TryGetValue(i, out j);
        System.Diagnostics.Debug.Print("{0}",j);
        j++;
    }
  }
}

Suppose that the TryGetValue method of the passed-in implementation of IDictionary never actually writes to j [not possible if it's written in C#, but possible if it's written in another language]. What should one expect the code to print?

Nothing in the C# standard requires that j be maintained when code leaves the if statement, but nothing requires that it be reset to zero between loop iterations. Mandating either course of action would impose additional costs in some cases. Rather than doing either, the Standard simply allows that when TryGetValue is called, j may arbitrarily hold zero or the last value it held when it was in scope. Such an approach avoids unnecessary costs, but would be awkward if code were allowed to see the value of j between the time it re-enters scope and the time it is written (the fact that passing an uninitialized variable as an out parameter to code written in another language will expose its value was likely unintentional).