邹邹

Question

getting the following error:

Error   1   The constraints for type parameter 'T' of method     
'genericstuff.Models.MyClass.GetCount<T>(string)' must match the constraints for type    
parameter 'T' of interface method 'genericstuff.IMyClass.GetCount<T>(string)'. Consider  
using an explicit interface implementation instead.

class:

 public class MyClass : IMyClass
 {
     public int GetCount<T>(string filter)
     where T : class
       {
        NorthwindEntities db = new NorthwindEntities();
        return db.CreateObjectSet<T>().Where(filter).Count();
       }
 }

interface:

public interface IMyClass
{
    int GetCount<T>(string filter);
}

score 6down voteaccepted · Accepted Answer

You are restricting your T generic parameter to class in your implementation. You don't have this constraint on your interface.

You need to remove it from your class or add it to your interface to let the code compile:

Since you are calling the method CreateObjectSet<T>(), which requires the class constraint, you need to add it to your interface.

public interface IMyClass
{
    int GetCount<T>(string filter) where T : class;
}

Er lopen hier best wat Nederlanders rond inderdaad! :) – Wouter de Kort Aug 7 '12 at 12:40 — Wouter de Kort, Aug 7 '12 at 12:40
hier in OZ wat minder :) thanks anyway – user603007 Aug 7 '12 at 12:59 — user603007, Aug 7 '12 at 12:59

score 2down vote · Answer 2

You either need to apply the constraint to the interface method as well or remove it from the implementation.

You are changing the interface contract by changing the constraint on the implementation - this isn't allowed.

public interface IMyClass
{
    int GetCount<T>(string filter) where T : class;
}

score 0down vote · Answer 3

You need to constrain your interface, too.

public interface IMyClass
{
    int GetCount<T>(string filter) where T : class;
}

邹邹

The type 'T' must be a reference type in order to use it as parameter 'TEntity' in the generic type or method 'System.Data.Linq.Table<TEntity>'

3 Answers

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up vote2down vote	You either need to apply the constraint to the interface method as well or remove it from the implementation. You are changing the interface contract by changing the constraint on the implementation - this isn't allowed. `public interface IMyClass { int GetCount<T>(string filter) where T : class; }`
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