AvalonEdit is the WPF-based text editor used in SharpDevelop. There are three builds of AvalonEdit included in this package: .NET 4.0, 4.5 and .NET Core 3.0.
Sample code for AvalonEdit the WPF-based text editor used in SharpDevelop.
Modify your App.xaml file so the StartupUri points to the AvalonEdit Sample window:
StartupUri="/Samples/AvalonEdit/Window1.xaml"