Difference in positioning a using block

Question

What is the difference between :

Case 1:

var reader = cmd.ExecuteReader();
using(reader)
{
     while(reader.read())
     {
         DoSomething(ref reader);
     }
}

Case 2 :

using(var reader = cmd.ExecuteReader())
{
     while(reader.read())
     {
         DoSomething(ref reader);
     }
}

Case 1 : Doesn't produce any compiler errors but a warning line is shown under the reader variable inside the using block and the warning says :

Possibly incorrect assignment to local 'variable' which is the argument to a using or lock statement. The Dispose call or unlocking will happen on the original value of the local.

http://msdn.microsoft.com/en-us/library/zhdyhfk6%28v=vs.90%29.aspx

Case 2 : It produces a compiler error, due to the restricting the using variable to readonly access.

Neglecting the need to use ref, does this mean that in Case 1, the reader variable won't be disposed, or at least disposed "as expected" ? Why is Case 1 doesn't produce any compiler errors but Case 2 does ?

Answer 1

Variables declared within a using statement are read-only (they cannot be assigned after initialization).
Therefore, you cannot pass them as ref parameters, since the point of ref parameters is to be assigned.

In your first example, the variable is not declared in the using statement, so it isn't read-only.

It gives a compiler warning for the same reason that the second variable is read-only.

The reason is that (as the warning states) the using statement will only dispose the variable's original value.