In the example you give, there's really not any difference.  The reference  
only ever is stored in one variable, and that variable could just as  
easily have been a StreamReader.

But since StreamReader and StringReader simply implement the abstract  
TextReader class without adding anything new, it's nice to use them as  
TextReaders.  The same thing would apply to any other class someone might  
derive from TextReader.  That way the code is more general-purpose and  
doesn't depend on a specific implementation of TextReader.  Of course,  
usually this would be a matter of passing the local variable to some  
method that takes a TextReader, and of course that can be done whether or  
not the variable is declared explicitly as just a TextReader versus a  
StreamReader.

Obviously at some point you need to be explicit about which class is  
created, but this only needs to be done in one specific place and the rest  
of the code can be more reusable by using TextReader instead.

To some extent, declaring the local variable as a TextReader just makes it  
that much more clear that the code within the using statement isn't  
dependent at all on the reader being a StreamReader.  Here, it's likely an  
issue of code readability as anything else.

Pete

Hi,

In that sentence using as another meaning, it basically do two things, first
it create a scope for the variable reader and second and more importantly it
call Dispose at the end of the scope.
In this construction you do not have to close the Reader. as the Dispose
method will do it for you, even if an exception is throw inside the scope.

Of course the variable being "used" must implement IDisposable

A StreamReader is a "type" of TextReader and that's why the code works.  If
you ask me for fruit and I pass you an apple, it is ok, because an apple is
a "type" of fruit.  This isn't quite polymorphism at work here, but simple
inheritance concepts.

Having said that, since a StreamReader is a more specific "type" than a
TextReader, you probably wouldn't want to keep this line of code as it is.
You'd want to declare reader as a StreamReader so that you can take full
advantage of a StreamReader's members in the using block.  As it is now,
you'd only be able to use the more generic members of a TextReader in the
using block even though you really do have a StreamReader in memory.

-Scott

Have you tried:

using (TextReader reader = new StreamReader(fullPathname))
{
 source.Text = reader.ReadToEnd();
}

Hilton