Problem:
When I wanted to build the solution in Visual Studio, I got message: Rebuild All failed.

Description:
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.
Solution:
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.
Summary:
The given project’s Target Framework version should be higher or equal than the target Framework version of referenced projects.