If you want to use any national characters set (i.e.
windows-1250) on .NET Core, thus also with FirebirdClient, you need to do some extra steps.
First you need to install
System.Text.Encoding.CodePages package, but that’s not enough. You then need to call
Encoding.RegisterProvider(CodePagesEncodingProvider.Instance); before you plan to use new encodings. In case of FirebirdClient before using i.e.
FbConnection (or related classes).
Without this your selection is limited (UTF-8, US-ASCII, UTF-16[BE] to name a few).