WCF Dynamic validation

For this I can suggest you 2 things

1.Maintain separate validation DLL, which consists of validations only based on the client, and if you want to combine all client's validations in a single validation dll you can use preprocessor directives for separation of clients base on Conditional compilation
Look the sample on preprocessor directives
Preprocessor directives in C Sharp[^]

2. Maintain parameter table in database for all clients, because the parameter validations may vary with each and every client, so you have to pick specific client parameters from table and validate it and return validated message to main solution

Thanks

SP