Error referencing Class Library in your Console app

Recently I referenced a C# class library DLL (with namespace starting as Com) from another project (Console App) and added Using in the program to get the namespace.

But the build failed as "The type or namespace name 'Com' could not be found (are you missing a using directive or an assembly reference?)"

I found out that the Project referencing the assembly may have different framework type from the assembly using it.

As it turned out, the Console app is set to ".Net Framework 4 Client Profile" by default. Changing it to ".Net Framework 4" resolved the issue.