Limiting User-Entered Text - Win32 Programmer's Reference

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Limiting User-Entered Text

The default limit to the amount of text a user can enter in an edit control is 32 kilobytes. An application can change the amount of text the user can enter by sending the control an EM_SETLIMITTEXT message. This message sets a hard limit to the number of bytes the user can enter into an edit control, but affects neither text that is already in the control when the message was sent nor text copied to the control by the SetDlgItemText function or the WM_SETTEXT message. For example, suppose that the application uses the SetDlgItemText function to place 500 bytes in an edit control, and the user also enters 500 bytes (1000 bytes total). If the application then sends an EM_SETLIMITTEXT message limiting user-entered text to 300 bytes, the 1000 bytes already in the edit control remain there, and the user cannot add any more text. On the other hand, if the application sends an EM_SETLIMITTEXT message limiting user-entered text to 1300 bytes, the 1000 bytes remain, but the user can add 300 more bytes.

When the user reaches the character limit of an edit control, Windows sends the application a WM_COMMAND message containing an EN_MAXTEXT notification message. This notification message does not mean that memory has been exhausted, but that the limit for user-entered text has been reached; the user cannot enter any more text. To change this limit, an application must send the control a new EM_SETLIMITTEXT message with a higher limit.

As an example of the use of EM_SETLIMITTEXT and EN_MAXTEXT, suppose that the application must limit the user to no more than four characters in an edit control. The application would use EM_SETLIMITTEXT to specify a four-character limit. If the user tried to enter a fifth character, Windows would send an EN_MAXTEXT notification message to the application.

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