Question

Am I doing something wrong or is it not possible to specify a generic class as a constraint to a generic method?

I have been playing around with generics and db4o (open source object database) and am writing a test program (see code below) to store and retrieve some user defined generic collections.

I am attempting to write a generic method (see GetCollectionFromDb below) to retrieve a specifically typed collection from the database. Unfortunately, the code below returns a compiler generated error for the line:

 MyCollection1 collection3 =                   GetCollectionFromDb<MyCollection1>(Collection1Name); 

The error message is:

The type 'GenericsTest.MyCollection1'cannot be used as type parameter 'T' in the generic type or method 'GenericsTest.Program.GetCollectionFromDb<T>(string)'. There is no implicit reference conversion from'GenericsTest.MyCollection1' to 'GenericsTest.MyCollectionBase<GenericsTest.MyCollection1>'. 

I would appreciate any suggestions as to what I may be doing wrong or how I could approach this differently to reach the deisred outcome.

using System; using System.Collections; using System.Collections.Generic; using System.Collections.ObjectModel; using Db4objects.Db4o;   namespace GenericsTest      {         public class Entity1         {             public string SomeProperty { get; set; }         }          public class Entity2         {             public string SomeProperty { get; set; }         }          public abstract class MyCollectionBase<T> : Collection<T>         {             protected MyCollectionBase() : this('') { }              protected MyCollectionBase(string pCollectionName)              {                 CollectionName = pCollectionName;              }              public string CollectionName { get; set; }         }          public class MyCollection1 : MyCollectionBase<Entity1>         {             public MyCollection1(string pCollectionName) :                                           base(pCollectionName) { }              public void DoSomeWorkOnCollection1() {}         }          public class MyCollection2 : MyCollectionBase<Entity2>         {             public MyCollection2(string pCollectionName) :                                           base(pCollectionName) { }              public void DoSomeWorkOnCollection2() { }         }          public class Program         {             public static IObjectContainer db = null;              public static void Main(string[] args)             {                 const string Collection1Name = 'Entity1Collection';                 const string Collection2Name = 'Entity2Collection';                 db = Db4oFactory.OpenFile('Test.db');                  Entity1 entity1 = new Entity1();                 MyCollection1 collection1 = new MyCollection1(Collection1Name);                 collection1.Add(entity1);                 db.Store(collection1);                  Entity2 entity2 = new Entity2();                 MyCollection2 collection2 = new MyCollection2(Collection2Name);                 collection1.Add(entity1);                 db.Store(collection2);                  db.Commit();                 db.Close();                 db = Db4oFactory.OpenFile('Test.db');                  MyCollection1 collection3 =                         GetCollectionFromDb<MyCollection1>(Collection1Name);             }              private static T GetCollectionFromDb<T>(string pCollectionName)                                                   where T : MyCollectionBase<T>             {                 IList<T> queryResult = db.Query((T c) =>                                           c.CollectionName == pCollectionName);                 if (queryResult.Count != 0) return queryResult[0];                  return null;             }         }      } 

Answer 1

Just follow the T:

    // ...     {        //...        MyCollection1 collection3 = GetCollectionFromDb<MyCollection1>(Collection1Name);      }      private static T GetCollectionFromDb<T>(string pCollectionName) where T : MyCollectionBase<T>     {         IList<T> queryResult = db.Query((T c) => c.CollectionName == pCollectionName);         if (queryResult.Count != 0) return queryResult[0];         return null;     } 

would translate into:

    private static MyCollection1 GetCollectionFromDb<MyCollection1>(string pCollectionName) where T : MyCollectionBase< MyCollection1 >     {         IList< MyCollection1 > queryResult = db.Query((MyCollection1 c) => c.CollectionName == pCollectionName);         if (queryResult.Count != 0) return queryResult[0];         return null;     } 

Which is not what you want since MyCollection1 derives off MyCollectionBase< Entity1 > and not MyCollectionBase< MyCollection1 >, which is why you got the error. If you want the constraint to work, you will probably have to use a second type identifier to express the type being used in the generic collection.