How to find Encoding for 1251 codepage

The .NET Framework/.NET Core supports a large number of character encodings and code pages. To retrieve an encoding that is present in the .NET Framework/.NET Core pass the EncodingProvider object to the Encoding.RegisterProvider method to make the encodings supplied by the EncodingProvider object available to the common language runtime. Microsoft Document Reference

System.Text.Encoding.RegisterProvider(System.Text.CodePagesEncodingProvider.Instance);

Correct, you will get different results on different machines if you use Encoding.Default.

If you want a specific codepage, you can use Encoding.GetEncoding:

Encoding encoding = Encoding.GetEncoding("windows-1251");

For .NET Core you also need to reference the System.Text.Encoding.CodePages package and then use Encoding.RegisterProvider:

Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);