Actually, if we think in terms how Postgres identify types int4, int8 and so on, then:

• int8 in C# it is long
• uint8 in C# it is ulong

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/integral-numeric-types

In the accepted answer int8 is mentioned as sbyte, but I hardly believe that anyone would define byte (even sbyte) as int8