最佳答案
I used the second solution of How to resolve "Could not find schema information for the element/attribute <xxx>"?
I.e. created a XSD with the button to create a scheme. I changed the stylesheet in app.config to app.xsd but now I get the warning:
The Global element 'configuration' has already been declared in app.xsd
Even when changing the name the warning is shown. Does anybody have a solution for this?