whatId when using custom object

Question

I am having an html email template which uses certain merge fields from a custom object. Now I am using following apex code to send certain emails to users.

Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();

        mail.setTargetObjectId(t.OwnerId); 
        mail.setTemplateId(et.Id);
        mail.setWhatId(t.Id); 
        mail.SaveAsActivity = false;
        Messaging.sendEmail(new Messaging.singleemailMessage[] {mail});

Code is saved without error but when I run it it says "can't use whatId when sending email to users". Can anyone explain me how to overcome this error? When I use a visualforce template then same code works fine.

Answer 1

You can get around this by grabbing the user's email address from their user record (you've already got their ID so that's a trivial query) and then using that with the setToAddresses method of SingleEmailMessage, then you shouldn't need to set the target object ID and as such will bypass this restriction.

Answer 2

It would seem that you can't set the WhatId (which you have) when the target object Id is a UserId. You can set it when the target is a contact or such. This post i found seems to confirm that hunch http://stackoverflow.com/questions/11105086/setwhatid-in-salesforce-using-email-template

Answer 3

This is a glaring hole that I'm sad to say has been here since the beginning. Particular frustrating since workflow alerts seem to have no problems merging a custom object template when sending to a user. You can work around it by creating a contact with the users email and then deleting it after the email has been sent.

private static void sendMyTemplate(Id userId, Id mergedObjectId) {
  User user = [select email, firstName, lastName from User where id = :userId];
  Contact tempContact = new Contact(email = user.email, firstName = user.firstName, lastName = user.lastName);
  insert tempContact;
  Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
  mail.setTargetObjectId(tempContact.id); 
  mail.setTemplateId(template.Id);
  mail.setWhatId(mergedObjectId); 
  Messaging.sendEmail(new Messaging.singleemailMessage[] {mail});
  delete tempContact;
}