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HOWTO: How to Prevent UI in an Out of Process Server

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This article was previously published under Q187909

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In an Out of Process Automation Server, it is possible to display a modal dialog box, Messagebox, error message or Wait Window. However, if you distribute the server remotely, you do not want to have any user interface display. To prevent a server from going into a modal state, issue the SYS(2335,0) function.

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More information

If any type of modal dialog appears in a remote server, the server processing stops and the client appears to hang until some type of user interaction handles the dialog. To prevent the server from going into a modal state, use the SYS(2335,0) function early in the code. This is the "Unattended Server Mode" and it causes Visual FoxPro to generate an error if any type of modal user interface (UI) is attempted in the server code. The error code is 2031:"User-interface operation not allowed at this time", which is a trappable error.

The following code is a sample of how the SYS(2335,0) prevents any UI from displaying in an out of process server.

Sample Code

   DEFINE CLASS sampclass AS custom OLEPUBLIC

      Name = "sampclass"

      Procedure Init
         *!* Turn on unattended mode as early as possible.

      PROCEDURE GenDisp
         *!* Display some type of UI, this could be anything from an error
         *!* message to a dialog box caused by an ODBC Logon.
         WAIT WINDOW "test"

      PROCEDURE Error
         LPARAMETERS nError, cMethod, nLine
         *!* Trap error and return the error message to client.
         returnstr = ALLTRIM(STR(nError)) + ': "'+MESSAGE()+'" on Line ' ;

To build this code, create a project and name it MySample. Copy and paste this code into a .prg file and add it to the project. Build the project as an EXE. To run the sample code from the Command window, issue the following:
   oX = Createobject("MySample.sampclass")
A Dialog box should display stating the 2031 error number, message, and line number where the error occurs.

The SYS(2335) function is only for Out of Process Servers (EXE's); the In Process servers (DLL's) are by default in unattended mode and can not display any UI.

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Visual FoxPro Help; search on: "SYS(2335)"; topic: "SYS(2335)- Unattended Mode"

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Keywords: KB187909, kbhowto

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Article Info
Article ID : 187909
Revision : 4
Created on : 3/1/2005
Published on : 3/1/2005
Exists online : False
Views : 362