Hi Ozcan Oksuz,
Thank you for contacting us.
_radDock.DockControl(window, DockPosition.Top, DockType.ToolWindow);
you wrap the window (ToolWindow instance) in a HostWindow. To access its Tag you could use the code:
tag = ((HostWindow)(_radDock.DockWindows.ToolWindows)).Content.Tag;
Here Content is your instance of ToolWindow.
For the specific case of using ToolWindow we recommend using the DockWindow method of RadDock:
In this case you can access the Tag of the window with the code:
tag = _radDock.DockWindows.ToolWindows.Tag;
Hope it helps.
the Telerik team
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