As section 2.2 of The OAuth 2.0 Authorization Framework says:
The authorization server issues the registered client a client
identifier — a unique string representing the registration
information provided by the client. The client identifier is not a
secret; it is exposed to the resource owner and MUST NOT be used
alone for client authentication. The client identifier is unique to
the authorization server.
The client identifier string size is left undefined by this
specification. The client should avoid making assumptions about the
identifier size. The authorization server SHOULD document the size
of any identifier it issues.
So you can define the client identifier by yourself. It depends your choice. You can use System.Guid to generate one simply, or use uid + systemTime, also you can Hash it, encrypt it or anything you want else.
But the client secret should be a cryptographically strong random string. You can generate one like this:
RandomNumberGenerator cryptoRandomDataGenerator = new RNGCryptoServiceProvider();
byte[] buffer = new byte[length];
cryptoRandomDataGenerator.GetBytes(buffer);
string uniq = Convert.ToBase64String(buffer);
return uniq;
Also you can use cryptographic hash functions() to hash UUID+SystemTime+somthingelse to implement it yourself.
If you want to know more practices, you can refer to some open implementations from here.