C# Error CS1542 – ‘{0}’ cannot be added to this assembly because it already is an assembly

C# Error

CS1542 – ‘{0}’ cannot be added to this assembly because it already is an assembly

Reason for the Error & Solution

‘dll’ cannot be added to this assembly because it already is an assembly; use ‘/R’ option instead

The file that was referenced with the compiler option was not built with module element of the ; use to reference the file in this compilation.