PRB: CS0518 Compiler Error When You Build a Visual C# .NET Application

When you build a Visual C# .NET application, you may receive the following error message in the compiler:

The following are possible causes:

• You may have specified the /nostdlib option from the command line compiler.
• The program may contain a reference to the incorrect Mscorlib.dll file.
• The Visual Studio .NET or .NET Framework Software Development Kit (SDK) may not be installed correctly on the system.
• Residual components from an earlier installation of the .NET Framework SDK that are incompatible with the latest installation may remain on the system.

To resolve this problem, follow these steps:

1. Make sure that you do not specify the /nostdlib option from the command line compiler.
2. Make sure that the project refers to the correct Mscorlib.dll file.
3. If steps 1 and 2 do not resolve the error, reinstall the .NET Framework SDK.

This behavior is by design.

The /nostdlib option prevents the import of Mscorlib.dll, which defines the entire System namespace. Use this option if you want to define or create a user-specific System namespace.