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The key point in the validation of an edit control is the interception of WM_CHAR message; This can be done by subclassing. When subclassing, the message queue of a control is preceeded with another message queue, that can handle the messages first. In the example of CHertzEdit the additional message queue rejects not only the entry of invalid characters, but also formats the content of the control. For the data transfer via DDX two additional methods must be added to the control that can set and get the content.