/target:winexe
The /target:winexe option causes the compiler to create an executable (EXE), Windows program. The executable file will be created with the .exe extension. A Windows program is one that provides a user interface from either the NGWS framework library or with the Win32 APIs.
Use /target:exe to create a console application.
Unless otherwise specified with the /out option, the output file name takes the name of the input file that contains the Main method.
When specified at the command line, all files until the next /out or /target option are used to create the Windows program.
One and only one Main method is required in the source code files that are compiled into a .exe. The /main compiler option lets you specify which class contains the Main method, in cases where your code has more than one class with a Main method.
Compile in.cs
into a Windows program:
csc /target:winexe in.cs