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Each document windows has it own buttons configuration. So you can disable the close button for all document windows using the code snippet below:
item.DocumentButtons = DocumentStripButtons.ActiveWindowList;
Keep in mind that the DocumentButtons
enumeration defines which buttons should be visible (for a DocumentWindow instance).
Should you have further questions, I would be glad to help.
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Check out the Windows Forms project converter, which aids the conversion process from standard Windows Forms applications written in C# or VB to Telerik UI for WinForms.For more information check out this blog post
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