Bill Blogs in C#

Over the past two weeks, I’ve gotten two questions relating to Windows Forms Databinding and custom collections.

In the first case, the developer had created a collection that held polymorphic items:

public class MyBaseClass
{
  // internals elided.
}

public class Derived1 : MyBaseClass
{
  // internals elided.
}

public class Derived2 : MyBaseClass
{
  // internals elided.
}

public class Derived3 : MyBaseClass
{
  // internals elided.
}

ArrayList myCollection = new ArrayList();
myCollection.Add( new Derived1());
myCollection.Add( new Derived2());
myCollection.Add( new Derived3());

// Use a data source doesn’t work:
MyDataGrid.DataSource = myCollection;

This does not work because the WindowsForms DataGrid class makes an assumption that all the objects in the collection are the exact same type. In the above example, it assumes all objects are of type Derived1. That fails. This person wanted the DataGrid to assume that it had a collection of MyBaseClass objects.

The second question involved navigation. (If you are not familiar with navigating in a datagrid, see this MSDN article) I had a colleague create a collection where one of the properties was itself a collection. For example:

public classCustomer
{
  public string Name
  {
    get{ return _name;}
  }

  // The contained collection:
  private ArrayList _orders;
  public IList Orders
  {
    return _orders;
  }
}

He wanted to bind a data grid to a list of customers and allow users to navigate to the set of orders for that customer. That did not work either.

Well, the answer to both questions is the same: Implement the ITypedList interface. ITypedList contains two methods:

public interface ITypedList
{
  public PropertyDescriptorCollection GetItemProperties(
    PropertyDescriptor[] listAccessors);
  public string GetListName( PropertyDescriptor[]
    listAccessors);
}

Both of these routines give information about the elements in the collection. GetItemProperties() returns a collection that describes all the properties that should be displayed in the collection. For a single level collection, these routines are always called with a null listAccessors parameter. That will solve the first problem. You simply return a name for your collection for GetListName:

public string GetListName( PropertyDescriptor[]
  listAccessors)
{
  return "MyBaseClass";
}

For GetItemProperties, you return a collection of all the properties you want displayed:

public PropertyDescriptorCollection
  GetItemProperties( PropertyDescriptor[]
  listAccessors)
{
  return TypeDescriptor.GetProperties(
    typeof( MyBaseClass ));
}

That solves the first problem. To solve the second problem, you need to understand the listAccessors parameter. This is an array of PropertyDescriptors that describe how to navigate from the top-level list to the specific collection currently being used. My simple example above had only one contained type. Therefore, the listAccessors parameter will either be null, or contain one element. That tells you what should be returned:

public string GetListName( PropertyDescriptor[]
  listAccessors)
{
  if ( null == listAccessors )
    return "Customers";
  else
  {
    PropertyDescriptor p = listAccessors[ 0 ];
    return p.DisplayName;
  }
}

The GetItemProperties() method is similar. You return the public properties of the type contained in the collection referenced by listAccessors[0]. In my simple example above, listAccessors[0] would be the type ArrayList, which is the type of the collection. You need to return the type of the objects in the collection (here that would be the Order class).

The whole point of this discussion is that the DataGrid uses reflection to determine how to bind to your data. If your datasource supports the ITypedList interface, those methods are used to support navigation and to determine which columns should be displayed. If not, the DataGrid displays all the properties found in the first item in the collection, and navigation is not supported.

You can learn more here: http://www.ftponline.com/vsm/2003_06/online/wagner/default_pf.aspx