ASP.Net validators and user controls (ascx) are great. They save us a lot of time.
But when we mix a User Control and a ASP.Net validator the following error occurs:
Control 'UserSelector' referenced by the ControlToValidate property of 'rvUserSelector' cannot be validated.
The solution to this problem is to use the attribute ValidationProperty on the User Control:
public partial class UserSelector : UserControl
In this example the UserID property of the User Control is used by the validators to perform the validation.
To end, I would like to thank you Alexandre Simões for his help in solving this issue.