Ensure the FastColoredTextBox.dll is present in your project's bin/Debug or bin/Release folder, or re-install via NuGet.
Due to its common use in script executors, the DLL may sometimes cause false-positive warnings from antivirus software. FastColoredTextBox.dll
It is designed for Windows Forms. To use in WPF, you must use WindowsFormsHost . Ensure the FastColoredTextBox