Web Forms :: File Not Found Error - Try / Catch?

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(?).