I have a table that will not allow multiple such as email.  Two people can't have the same email address, when I put in the same email address, I get an error returned from the "A critical error has occured. Exception has been trhown by the target of an invocation." 

I need to be able to take that error and just dispay a message saying that name is already in the database, please enter another.

How would I handle this?

Thanks

If this is for users, and you want to restrict users to one username per email address, then you can modify the settings for the Membership Provider in your web.config file.

Change: requiresUniqueEmail="false"

To: requiresUniqueEmail="true"

I should add that if the error is coming from another module, you'll have to modify the code for that module to handle the exception.

I guess I should have a little clearer.  I have a table  that holds the UserID and an associated sub domain on my web site.  Its a module on building landing sites, but we can only have one subdomain per site no matter who puts it in.  First come first serve.

I have SQL set not to allow duplicates in the website column.  Which works out great, I now need to know how to handle the error that SQL sends me saying "duplicate value".

Hope that helps. 

Do you know how to use try,catch,finally blocks of code?

In c# you would write:

try
{
my code goes here
}
catch
{
code to execute if there's an exception generated in the try code
}
finally
{
cleanup code goes here and is executed if try succeeds or if catch is called.
usually this is for cleanup ..
}