I am validating a foreign key entered in a text box. I already have a rule created by the business layer generator for a required entry. Now I want to make sure that the number matches a row in the foreign table. I would like some advice on best practice here. Currently, I have modified that validation rule to add code to check for a valid entry.

                int PatientCount = ExecuteScalar("SELECT COUNT(*) FROM Patient WHERE ClientPatientId = '" + ClientPatientId + "'");
                if (PatientCount = 0)
                {
                    Msg = "Unable to locate this patient.";
                    AddErrorProviderBrokenRule("ClientPatientId", Msg);
                }

If it is valid, I want to look up and display some related fields. I was thinking that I could use the event,
txtClientPatientId_TextChanged(object sender, EventArgs e), and have it call the foreign business object. The only problem is that it fires after each letter entered, doesn't it? Is this a reasonable approach to doing this? Is there a better solution?

Thanks.