如何使用C＃中的对象初始化程序初始化包装在getter / setter中的EntitySet？

Question

I am adapting the Windows Phone Database Example using MVVM for my own app. Here it suggests wrapping the EntitySet properties on the model objects with a getter/setter combination that allows assignment using an IEnumerable (using the Assign method) eg

// Example method from a model class 'SomeModelObject'
[Association(Storage = "_todos", OtherKey = "_categoryId", ThisKey = "Id")]
public EntitySet<ToDoItem> ToDos
{
    get { return this._todos; }
    set { this._todos.Assign(value); }
}

However when I try and instantiate an object that has an EntitySet property it will not allow it, eg

SomeModelObject myModelObject = new SomeModelObject() {
    Property1 = "foo",
    Property2 = true,
    // Following raises an error, even though setter should allow assignment
    // from an IEnumerable (because of the use of 'Assign' in the setter)
    ToDos = new List<ToDoItem>() {
        new ToDoItem(),
    },
};

The error is as follows,

Error    1   Cannot implicitly convert type 
'System.Collections.Generic.List<SomeApp.ToDoItem>' to 
'System.Data.Linq.EntitySet<SomeApp.ToDoItem>'

How do I instantiate objects referenced from an EntitySet ?

Answer 1

Well, there are two problems with the code you've posted.

I suspect the main problem is that you're trying to set the value of the property to a List<ToDoItem> when the type of the property is EntitySet<ToDoItem> . (The error message you've posted doesn't match the code you've posted, but never mind.) You say it "should allow assignment from an IEnumerable , but I don't know why you'd expect that to be the case, given the declaration of the property:

public EntitySet<ToDoItem> ToDos { ... }

The property is not of type IEnumerable<ToDoItem> .

You're also using new SomeModelObject twice for some reason:

SomeModelObject myModelObject = new SomeModelObject() {
    new SomeModelObject() {
        Property1 = "foo",
        ...
    }
};

I'll assume your real code looks more like this:

SomeModelObject myModelObject = new SomeModelObject() {
  Property1 = "foo",
  ... 
};

Please be more careful in future - it's very hard to diagnose code that isn't posted.

There are three options around the EntitySet .

If you want to add new items to an existing EntitySet<ToDoItem> you can do that within the object initializer:

SomeModelObject myModelObject = new SomeModelObject {
  Property1 = "foo",
  Property2 = true,
  ToDos = {
    new ToDoItem(),
  }
};

Alternatively, if you want to set the value of the property itself, you need to create a new EntitySet<ToDoItem> :

SomeModelObject myModelObject = new SomeModelObject {
  Property1 = "foo",
  Property2 = true,
  ToDos = new EntitySet<ToDoItem> {
    new ToDoItem(),
  }
};

This seems unlikely to be a good idea though, and I'd actually suggest making the setter private.

Alternatively, if you really want to be able to assign any IEnumerable<ToDoItem> , you can just change the type of the property:

[Association(Storage = "_todos", OtherKey = "_categoryId", ThisKey = "Id")]
public IEnumerable<ToDoItem> ToDos
{
    get { return this._todos; }
    set { this._todos.Assign(value); }
}