Is a generic class used as a constraint for a generic method in C #?
Am I doing something wrong or unable to define a generic class as a constraint on a generic method?
I have been playing around with generics and db4o (open source database) and I am writing a test program (see code below) to store and retrieve specific custom generic collections.
I'm trying to write a generic method (see GetCollectionFromDb below) to retrieve a specially injected collection from a database. Unfortunately, the code below returns a compiler generated error for the string:
MyCollection1 collection3 =
GetCollectionFromDb<MyCollection1>(Collection1Name);
Error message:
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 might be doing wrong, or how I might approach this in a different way to achieve the worst result.
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;
}
}
}
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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;
}
translates 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;
}
This is not what you want, since MyCollection1 outputs MyCollectionBase <Entity1> and not MyCollectionBase <MyCollection1>, which is why you got the error. If you want the constraint to work, you probably have to use a second type identifier to express the type used in the shared collection.
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Your type does not satisfy the constraint. You provided MyCollection1
, which comes from MyCollection<Entity1>
. However, this does not mean that it comes from MyCollection<MyCollection1>
.
Perhaps you want to express the constraint on two type parameters rather than one:
private static T GetCollectionFromDb<T, U>(string pCollectionName)
where T : MyCollectionBase<U>
Then call it with
GetCollectionFromDb<MyCollection1, Entity1>(Collection1Name);
If that doesn't do the trick, please let us know why.
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