When I wanted to build the solution in Visual Studio, I got message: Rebuild All failed.
I have 2 projects in the solution. MainApplication and ClassLibrary. The MainApplication has reference to ClassLibrary.
The Error List shows no error. There is also no error in code.
Then I switched to Output window and here I got more informations.
From the Output you see, that the ClassLibrary was successfully built. Problem was during the building of the MainApplication.
warning MSB3274: The primary reference "D:\Temp\13\MySolution\ClassLibrary\bin\Debug\ClassLibrary.dll" could not be resolved because it was built against the ".NETFramework,Version=v4.7.2" framework. This is a higher version than the currently targeted framework ".NETFramework,Version=v4.6.1". 2>D:\Temp\13\MySolution\MainApplication\Form1.cs(25,23,25,35): error CS0103: The name 'ClassLibrary' does not exist in the current context
From this information we know that the MainApplication has targeting Framework v4.6.1 but the ClassLibrary has higher version v4.7.2.
I changed the Target Framework from v4.6.1 to v4.7.2 in MainApplication Project Properties.
After this change the Visual Studio successfully rebuilt the projects.
The given project’s Target Framework version should be higher or equal than the target Framework version of referenced projects.