There's no LanguageDefinition for C#

[orioldedios]

What can I do if I need this Text Editor in c#?

[BalazsJako]

You have to implement it. Try to use the current C/C++ support as a starting point, and re-use parts of it, and make something similar. It is way better than the slow std::regex-based language definitions.

[VerticalVeith]

If you still need C# support it might be worth the while to look at the Roslyn APIs.