Question

If I am using a try/catch/finally block where and how should I initialize variables? For example say I’m trying to use a FileStream . I want to catch any exceptions thrown while creating or using the stream. Then regardless of whether there were any problems or not I want to ensure any stream created is closed.

So I’d do something like this:

        System.IO.FileStream fs;
        try
        {
            fs = new System.IO.FileStream("C:\test.txt", System.IO.FileMode.Open);
            //do something with the file stream
        }
        catch (Exception exp)
        {
            //handle exceptions
        }
        finally
        {
            //ERROR: "unassigned local variable fs"
            if (fs != null)
            {
                fs.Close();
            }
        }

However this gives me an error in the finally block saying unassigned local variable fs. Yet, if I change the declaration of fs to System.IO.FileStream fs = null it works.

Why do I need to explicitly set fs to null? I’ve also tried declaring fs in the try block, but then I get the error The name fs does not exsist in the current context in finally block.

BTW: I know I could use a Using block, but the point of my question is to understand the correct usage of a try/catch/finally block.

Answer 1

See section 5.3 of the specification.

http://msdn.microsoft.com/en-us/library/aa691172(VS.71).aspx

At a given location in the executable
code of a function member, a variable
is said to be definitely assigned if
the compiler can prove, by static flow
analysis, that the variable has been
automatically initialized or has been
the target of at least one assignment.

With your try/catch/finally, the assignment of the try block cannot be guaranteed when you attempt to access the object in the finally block. As you have seen, you can meet the requirement by assigning an initial value to the variable (null, in this case).