MSDN: System.Security.Cryptography.HMACSHA256.Create()
SHA-1 HMAC. , Create (String), .
Create , 'Create function.
Try:
System.Security.Cryptography.HMAC hmac2 = System.Security.Cryptography.HMACSHA256.Create("System.Security.Cryptography.HMACSHA256");
, , ... .
:
System.Security.Cryptography.HMAC hmac2 = System.Security.Cryptography.HMAC.Create("System.Security.Cryptography.HMACSHA256");