file not found error...try/catch?

I'm new to c# and ASP.Net. I come from a VB environment. I'm looking at 2 different file extensions. If one doesn't exist look at the next. I was hoping to use the try/catch function. I can get it to find the first extension but not the second. Here is my the try/catch part of code:
try
{
     string website = xAll;
     Response.Redirect(website);
}
catch (FileNotFoundException)
{
     string website1 = xAll1;
     Response.Redirect(website1);
}
The error dialog that pops up shows Connection Error in the title and a yellow exclamation point with File Not Found!
I thought the FileNotFoundException would work(?).
Thanks in advance
...