How better check Request.QueryString string parameter for null?

Question

I need explanations.. I using C#.NET to web applications, I always write:

 string val = Request.QueryString["foo"];

and then

if(!string.IsNullOrEmpty(val)) 

What's the difference:

string val = Request.QueryString["foo"];

I was advised to do:

string val = Request.QueryString["foo"] as string;
if(!string.IsNullOrEmpty(val)) 

What's the difference?

Answer 1

The first is better:

string val = Request.QueryString["foo"];

The second version returns null if the result of the call is not a string, but you know it always will be a string because the QueryString member has type NameValueCollection . The indexer is defined to return a string :

public class NameValueCollection : NameObjectCollectionBase
{
    // ...
    public string this[string name] { get; set; }
    // ...
}

Answer 2

The as string is redundant as Request.QueryString["foo"] already is a string. (So there is no other difference than the second makes you look like you don't know your framework ;-) )