C# Access the Properties of a Generic Object

Question

I have a method that counts the number of Contacts each Supplier, Customer and Manufacturer has (this is a scenario to try make explaining easier!)

The models are all created by Linq to SQL classes. Each Supplier, Customer and Manufacturer may have one or more Contacts

public int CountContacts<TModel>(TModel entity) where TModel : class
{
    return entity.Contacts.Count();
}

The above of course doesnt work, because the 'entity' is generic and doesnt know whether it has the property 'Contacts'. Can someone help with how to achieve this?

Answer 1

An easy way would be to attach an interface to the classes being implemented in the generic.

public int CountContacts<TModel>(TModel entity) where TModel : IContacts


interface IContacts
{
   IList<Contact> Contacts {get;} //list,Ilist,ienumerable
}

Answer 2

One way to impose a contract where Suppliers, Customers and Manufactors must contain a Contacts property is with interfaces. Make each entity implement one interface that contains the Contacts property:

interface IContactable
{
   IEnumerable<Contact> Contacts {get;}
}


public int CountContacts<TModel>(TModel entity) where TModel : class, IContactable
{
    return entity.Contacts.Count();
}

Answer 3

Another way is to create an interface just for counting. You can call it ICountable.

From MSDN

public interface ICountable<out T>  
{
    int Count{ get; }
}
public class MyCollection : ICountable<string>, ICountable<FileStream>
{  
    int ICountable<string>.Count
    {
        get { return 1; }
    }
    int ICountable<FileStream>.Count
    {
        get { return 2; }
    }
}

Answer 4

All of the answers so far are correct but should also point out that the reason your code doesn't compile is because the TModel types don't have anything in common. By specifying a common base-class or interface they all implement with your 'Contacts' property your code will work.

Answer 5

There are several solutions to the problem.

• Inherit the same abstract class or implement the same interface with the entities. (The others exhausted every possible solution with this.)
• If you are using .NET 4, the dynamic keyword may be the cheapest solution.

For example:

public int CountContacts(dynamic entity)
{
    return entity.Contacts.Count();
}

This means that entity will not be evaluated until runtime, and if you happen to call the method on an object that doesn't have a Contacts property, it will throw you an exception.