The /target:library option causes the compiler to create a dynamic-link library (DLL) rather than an executable file (EXE).
/target:library
Remarks
The DLL will be created with the .dll extension.
Unless otherwise specified with the /out option, the output file name takes the name of the first input file.
When specified at the command line, all files up to the next /out or /target:module option are used to create the .dll file.
When building a .dll file, a Main method is not required.
To set this compiler option in the Visual Studio development environment
Open the project's Properties page.
Click the Application property page.
Modify the Output type property.
For information on how to set this compiler option programmatically, see OutputType.
Example
Compile in.cs, creating in.dll:
csc /target:library in.cs