C# Generic study

Generic Constraints

Generic Constraints is used when T in the class needs to have specific characteristics/methods that object type doesn't have (object type has only "toString" and "hasHashCode" methods).

• "where" keyword can be used to constrain the generic type, for example: 
• public class SqlRepository<T> : IRepository<T> where T : class
• we can create a new interface with such method and use "where" keyword to constrain T. for example:
• public interface IEntity {bool IsValid()} so that "where T : IEntity" will allow "T.IsValid()".
• Note. usually we do not need to define own interface, check in advance if .net framework already has such!
• default(T) will assign default value to T. if T is class -> null; if T is struct -> 0
• where T : new() -> T has a default constructor so that T can be instantiated in code, for example:
• public T CreateNewT() {T t = new T(); return t;}
• constraints is preferable to be implemented in concrete class instead of in interface
• Covariant V.S. Contravariant

 public interface IReadOnlyRepository<out T> : IDisposable  
   {  
     T FindById(int id);  
     IQueryable<T> FindAll();  
   }  
   public interface IWriteOnlyRepository<in T> : IDisposable  
   {  
     void Add(T newEntity);  
     void Delete(T entity);  
     int Commit();  
   }  
   public interface IRepository<T> : IReadOnlyRepository<T>, IWriteOnlyRepository<T>  
   {  
   }  

 using (IRepository<Employee> employeeRepository   
         = new SqlRepository<Employee>(new EmployeeDb()))  
       {   
         AddManagers(employeeRepository);   
         DumpPeople(employeeRepository);    
       }  

• Reflection in C# (for Generic part)  // ToDo