Howtoforge published a tutorial about installing Mono on Debian 11.
How to Install Mono on Debian 11
Mono is an open-source platform for developing applications and libraries. It has been developed by Novell Inc. since 2002, with a focus on the .NET framework developed by Microsoft. Mono provides a complete CLR (Common Language Runtime) implementation that can run class libraries or components that target Microsoft's original .NET runtime.