Declaring and initialising a variable in a using block

Question

After a bit of advice here, regarding best practice coding - I am still a beginner with regards to C#. I have been reading on how using blocks are essential for disposing objects and attempting to implement these.

My question however, is if you are declaring and initialising a variable, should this be in a using block also?

I have tested and it seems to compile fine. But would appreciate peoples thoughts on this.

Thanks,

Answer 1

Using blocks are only helpful with objects that implement the IDisposable interface. It roughly translates to having a try...finally block that disposes the object in the finally. You can see more detail here.

Answer 2

It is common practice to declare a variable closest to where you need it. If it not used outside of the using block (or any other block) there's usually no reason to declare it outside.

Answer 3

The answer to your question "Should this be in the using block also?" is truly a matter of scope. Variables declared in the scope of your block (try, using, etc) will fall out of scope when the block is exited.

Consider the following two examples:

1. Declare variable, assign in block, exit block, require variable.

   WebHeaderCollection headers;
   
   using (var cl = new WebClient())
   {
       headers = cl.ResponseHeaders;
   }
   
   // headers is still in scope
   foreach (var k in headers.AllKeys) 
       Debug.WriteLine("{0} : {1}", k, headers[k]);
   

2. Declare and assign variable in block, exit block, require variable

   using (var cl = new WebClient())
   {
       var headers = cl.ResponseHeaders;
   
       // use headers here, just fine
       foreach (var k in headers.AllKeys) 
           Debug.WriteLine("{0} : {1}", k, headers[k]);
   }
   
   // headers is no longer in scope
   foreach (var k in headers.AllKeys) 
       Debug.WriteLine("{0} : {1}", k, headers[k]);
   

Option #2 throws an exception. So, first determine the scope requirement of your variable and then decide if you need it after the using block ends.

Answer 4

No. Putting any variable inside a using is just not necessary. It's basically help for the RAII idiom, which in .NET is commonly useful only for objects that wrap / use an unmanaged resource. And the standard pattern for those objects is to implement a IDisposable. The using keyword is just some nice syntax sugar for

try{ 
  //initialize and use variable 
} 
finally { 
  //cast to IDisposable - if not null, call Dispose()
}

So even though it will compile and work, it's no use at all if your object doesn't implement IDisposable.