Can I write programs in C# on Ubuntu?
Yes you can use mono
For a nice IDE to work in try monodevelop:
sudo apt-get install monodevelop
This will also pull in all the necessary runtimes needed.
What you are looking for is Mono, which is, quoting from the project's website:
An open source, cross-platform, implementation of C# and the CLR that is binary compatible with Microsoft.NET
Mono also comes with an Integrated Development Environment (IDE): MonoDevelop. Both softwares are packaged for Ubuntu.
Mono
MonoDevelop
This last package depends on the first one. Installing this package will automatically install the other.
You can develop C# applications on linux with .NET Core: https://www.microsoft.com/net/core
After you install it, type dotnet new in your terminal to get a list of application templates which can be generated for you to start.
As of .NET Core 2.0 today, this is the list:
Templates Short Name Language Tags
--------------------------------------------------------------------------------------------------------
Console Application console [C#], F#, VB Common/Console
Class library classlib [C#], F#, VB Common/Library
Unit Test Project mstest [C#], F#, VB Test/MSTest
xUnit Test Project xunit [C#], F#, VB Test/xUnit
ASP.NET Core Empty web [C#], F# Web/Empty
ASP.NET Core Web App (Model-View-Controller) mvc [C#], F# Web/MVC
ASP.NET Core Web App razor [C#] Web/MVC/Razor Pages
ASP.NET Core with Angular angular [C#] Web/MVC/SPA
ASP.NET Core with React.js react [C#] Web/MVC/SPA
ASP.NET Core with React.js and Redux reactredux [C#] Web/MVC/SPA
ASP.NET Core Web API webapi [C#], F# Web/WebAPI
global.json file globaljson Config
Nuget Config nugetconfig Config
Web Config webconfig Config
Solution File sln Solution
Razor Page page Web/ASP.NET
MVC ViewImports viewimports Web/ASP.NET
MVC ViewStart viewstart Web/ASP.NET
The current latest version is 2.0 and covers all my needs.