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Nachdem sich die Dialoge garnicht mehr öffnen liesen und die Schlussfolgerung in richtung Intellisense gingen, gab ich das Problem an MS. Hier gab es folgenden Workarround der soweit auch funktioniert:
We finally found a workaround that allows opening the forms in the WinForms designer (still building /clr): (1) do a build, (2) close the IDE, (3) delete the DMSKV.NCB (or, all NCBs, but in this case this is the only interesting one), (4) re-open the IDE, (5) wait for NCB population to complete. To do this, go to Class View and open the top node.
You'll see a line like "36 files to parse". When that goes to zero and real data is displayed, then the parsing is complete. Unfortunately, this only works for the initial NCB file population. And in this test case, you'll need to wait several times. So, I wait until the "Updating Intellisense..." message in the status bar disappears AND the CPU usage on the machine goes to zero. To do this, start Task Manager and minimize it. It will display CPU usage as an icon in the tray, near the clock, by default in the lower-right corner. (6) Open a form. This should work. (7) Once again, wait for NCB update to complete. (8) Open another form. (9) Wait for NCB update to complete. Etc.
If at any time you don't wait completely for the NCB update to occur, and the form displays the WSOD error, the form won't open successfully again, and you'll need to delete the NCB again.