Question

Is it possible to create generic restriction in C# using where to select only classes, who have Field with some name.

for example, I have AbstractService<T>
and I have a method IEnumerable<T> ProvideData(userId);

inside provide data I should select only instances with the same user bla-bla-bla.Where(d => d.UserId == userId). But d.UserId could not be resolved. How it possible to resolve this?

IMPORTANT: I can’t inherit T from class or interface, which have UserID field.

Answer 1

An interface is what your are looking for:

public interface IWithSomeField
{
     int UserId { get; set; }
}

public class SomeGenericClasss<T> 
  : where T : IWithSomeField
{

}

public class ClassA : IWithSomeField // Can be used in SomeGenericClass
{
     int UserId { get; set; }
}

public class ClassB  // Can't be used in SomeGenericClass
{

}

[Edit] As you edited your question to state you cannot change class to implement an interface, here is some alternatives, but none relies on generic constraint :

1. Check the type in the constructor :

code :

public class SomeClass<T>{
    public SomeClass<T>()
    {
        var tType = typeof(T);
        if(tType.GetProperty("UserId") == null) throw new InvalidOperationException();
    }
}

1. Use code contract invariant (not sure about the syntax) :

code :

 public class SomeClass<T>{
    [ContractInvariantMethod]
    private void THaveUserID()
    {
        Contract.Invariant(typeof(T).GetProperty("UserId") != null);
    }
}

1. Extend existing classes with partial classes

If your source classes are generated, you can cheat. I used this technique with lots of Web References having the same kind of parameter objects

Imagine the Web references produced this proxy code :

namespace WebServiceA {

    public class ClassA {
        public int UserId { get; set; }
    }
}
namespace WebServiceB {

    public partial class ClassB {
        public int UserId { get; set; }
    }
}

You can wrap them using in your own code:

public interface IWithUserId
{
    public int UserId { get; set; }
}
public partial class ClassA : IWithUserId 
{

}
public partial class ClassB  : IWithUserId
{

}

then, for your service, you can instantiate AbstractService for any of the Class of the several web services :

public class AbstractService<T> where T : IWithUserId
{
}

This technique works great but only applies when you can extend class in the same project because of the partial keyword trick.