Clean way to do an upsert (update or insert) operation in Dynamics CRM 4

Currently, I'm using the following code to perform an 'upsert' in CRM

try
{
 crm.Create(c);
} 
catch (SoapException)
{
 crm.Update(c);
}

Am I right in assuming that this type of updating will wipe all existing information from my business entity? And if so, how can I get the entity to update from the repository? Do I need fetch, retrieve, or something like that?

Thanks.

Solution

Using exception handling for flow control is bad 99.99% of the time; in this case among other things because you never know what the actual reason for your SoapException is.

A much cleaner way would be to check whether your record's ID field has a value; if so, do an Update, if not, do a Create (and maybe add the resulting ID to your object if you use it further). (We've seen a Create on a record with an ID actually do an update years ago, but we've never been able to reproduce it.)

Other than that, @ckeller is perfectly right; an attribute that is null in your object (because it wasn't in the ColumnSet when retrieving from the database or hasn't been set otherwise) will not be touched in an Update.