SQL connection in static class and wrapper
The class being static is not the only thing that makes testing more difficult then necessary.


var connectionString = ConfigurationManager.ConnectionStrings["appDatabase"].ConnectionString;



The hardcoded connection string name should be a parameter. You can have default constructor using a default connection string name but it's always a good idea to have another constructor that lets you use another value.


AddClient



If it's a ClientDatabase for CRUD about client objects then I find the client in name AddClient method redundant. Simply Add, Delete etc would be more appropriate.

By following this pattern you can have a single crud-generic interface for all types:

public interface IDataAccess&lt;T&gt;
{
    void Add(T obj);
    void Delete(T obj);
    ..
}


and later

public class ClientDataAccess : IDataAccess&lt;Client&gt;
{
    public void Add(Client obj) {..}
    public void Delete(Client obj) {..}
}