问题
I have been working on a project in Visual Studio, C# language and I found an error described above. How can I remove this error?
"Either 'GDI', 'WPF' or 'SILVERLIGHT' must be defined"
回答1:
Open the project properties for the project that throws the error (Alt+Enter in VS Solution Explorer), go to the Build tab and enter the symbol you want into the "Conditional compilation symbols" field. Enter one of the three values.
It seems the value got lost. I cannot tell you which value got lost. Check one of the other PDFsharp projects (I hope not all projects lost their symbols).
来源：https://stackoverflow.com/questions/31294307/either-gdi-wpf-or-silverlight-must-be-defined-how-to-remove-this-error