How do I specify what build configuration to use generally?

[fschwiet]

I have some C# projects that optionally include files based on the build configuration. When I open one of these files, I get an error message "The active document is not part of the open workspace. Not all language features will be available." Also the file contents are not available when searching by symbol. I have tried adding "--configuration", "Test" to the args: [] of the build task in tasks.json, but that doesn't help.