How to check that Request.QueryString has a specific value or not in ASP.NET?

I have an error.aspx page. If a user comes to that page then it will fetch the error path in page_load() method URL using Request.QueryString["aspxerrorpath"] and it works fine.

But if a user directly accesses that page the it will generate an exception because aspxerrorpath is not there.

How can I check that aspxerrorpath is there or not?

You can just check for null:

if(Request.QueryString["aspxerrorpath"]!=null)
{
   //your code that depends on aspxerrorpath here
}

Check for the value of the parameter:

// .NET < 4.0
if (string.IsNullOrEmpty(Request.QueryString["aspxerrorpath"]))
{
 // not there!
}

// .NET >= 4.0
if (string.IsNullOrWhiteSpace(Request.QueryString["aspxerrorpath"]))
{
 // not there!
}

If it does not exist, the value will be null, if it does exist, but has no value set it will be an empty string.

I believe the above will suit your needs better than just a test for null, as an empty string is just as bad for your specific situation.