How to define a strongly typed collection property

Description:
I have an object A with a one-to-many association with another object B. How can I have a collection of object B strongly typed?

Answer:
First of all, lets define an object that will be used as a child within the one-to-many collection. Call it "Role":

C#
public class Role : XPObject {  
    [Association("User-Role")]  
    public User User;  
    public string Name;  
}  

Visual Basic
Public Class Role  
    Inherits XPObject  
    <Association("User-Role")> _  
    Public User As User  

    Public Name As String  
End Class  

Now, we need to introduce a master object (lets call it user) that should contain a strongly typed collection of Role objects. Since version 6 of XPO is based upon .Net Framework 2.0 Generics technology, we can easily implement the required functionality using a generic XPCollection<T> generic collection class, as shown below:

C#
public class User : XPObject  
{  
    [Association("User-Role", typeof(Role))]  
    public XPCollection<Role> UserRoles {  
        get  
        {  
            return GetCollection<Role>("UserRoles");  
        }  
    }  
    public string FullName;  
}  

Visual Basic
Public Class User  
    Inherits XPObject  
    <Association("User-Role", GetType(Role))> _  
    Public ReadOnly Property UserRoles() As XPCollection(Of Role)  
        Get  
    Return GetCollection(Of Role)("UserRoles")  
        End Get  
    End Property  

    Public FullName As String  
End Class  

To implement a similar functionality with an old version of .Net Framework and our product, we'll have to create a custom collection that will provide a typed indexed property. Here is a code demonstrating this approach:

C#
public class User : XPObject {  
    public override void Reload() {  
        base.Reload();  
        Roles.Reload();  
    }  
    public User() : base() {  
        Roles = new RolesCollection(this, "Roles");  
    }  
    [Association("User-Role", typeof(Role))]  
    public RolesCollection Roles;  
    public string FullName;  
}  

public class RolesCollection : DevExpress.Xpo.Helpers.XPOneToManyCollection {  
    public RolesCollection(IXPSimpleObject owner, string collectionPropertyName) :  
        base(owner, owner.ClassInfo.GetMember(collectionPropertyName)) {}  
    public int Add(Role role) {  
        return base.Add(role);  
    }  
    public new Role this[int index] {  
        get { return (Role)base[index]; }  
    }  
}  

Visual Basic
Imports DevExpress.Xpo  

Public Class User  
    Inherits XPObject  

    Public Sub New()  
        MyBase.New()  
        Roles = New RolesCollection(Me, "Roles")  
    End Sub  

    Public Overrides Sub Reload()  
        MyBase.Reload()  
        Roles.Reload()  
    End Sub  

    <Association("User-Role", GetType(Role))> _  
    Public Roles As RolesCollection  

    Public FullName As String  
End Class  

Public Class RolesCollection  
    Inherits Helpers.XPOneToManyCollection  

    Public Sub New(ByVal owner As IXPSimpleObject, ByVal collectionPropertyName As String)  
        MyBase.New(owner, owner.ClassInfo.GetMember(collectionPropertyName))  
    End Sub  

    Public Shadows Function Add(ByVal role As Role) As Integer  
        Return MyBase.Add(role)  
    End Function  

    Public ReadOnly Property Item(ByVal index As Integer) As Role  
        Get  
            Return CType(MyBase.Object(index), Role)  
        End Get  
    End Property  
End Class