Question

I am using Implicit Data Templates which are applied to the items of an ItemsControl:

<ItemsControl ItemsSource="{Binding Path=CategoryAttributeVMs}"/>

<DataTemplate DataType="{x:Type my:CategoryAttributeDateFieldVM}">
    <DockPanel>
        <.....>
        <Button Command="{Binding Path=MainViewModel.CopyToChildrenCommand,
                                  Source={StaticResource Locator}}"
                CommandParameter="{Binding Mode=OneWay}">
    </DockPanel>
</DataTemplate>

<DataTemplate DataType="{x:Type my:CategoryAttributeIntegerFieldVM}">
    <DockPanel>
        <.....>
        <Button Command="{Binding Path=MainViewModel.CopyToChildrenCommand,
                                  Source={StaticResource Locator}}"
                CommandParameter="{Binding Mode=OneWay}">
    </DockPanel>
</DataTemplate>

This is databound to my ViewModel like so:

public abstract class CategoryAttributeVM
{
    protected CategoryAttributeVM(ICategoryAttribute categoryAttribute)
    {
        _categoryAttribute = categoryAttribute;
    }

    private readonly ICategoryAttribute _categoryAttribute;

    public string Name { get { return _categoryAttribute.Name; } }
    public object Value { get { return _categoryAttribute.Value; } }
}

public abstract class CategoryAttributeVM<T> : CategoryAttributeVM
{
    protected CategoryAttributeVM(ICategoryAttribute<T> categoryAttribute)
        : base(categoryAttribute) { _categoryAttribute = categoryAttribute; }

    private readonly ICategoryAttribute<T> _categoryAttribute;

    public new T Value
    {
        get { return _categoryAttribute.Value; }
        set { _categoryAttribute.Value = value; }
    }
}

public class CategoryAttributeDateFieldVM : CategoryAttributeVM<DateTime?>
{
    public CategoryAttributeDateFieldVM(ICategoryAttributeDateField categoryAttributeDateField)
        : base(categoryAttributeDateField) { }
}

public class CategoryAttributeIntegerFieldVM: CategoryAttributeVM<Int32?>
{
    public CategoryAttributeIntegerFieldVM(ICategoryAttributeIntegerField categoryAttributeIntegerField)
       : base(categoryAttributeIntegerField) { }
}

public class MainViewModel : ViewModelBase
{
    public MainViewModel()
    {
        CategoryAttributeVMs = new List<CategoryAttributeVM>( new CategoryAttributeVM[]
            {
                new CategoryAttributeDateFieldVM(new CategoryAttributeDateField("DateField", DateTime.Now)),
                new CategoryAttributeIntegerFieldVM(new CategoryAttributeIntegerField("IntField", 123))
            });

        CategoryAttributeVM2s = new List<CategoryAttributeVM>(new CategoryAttributeVM[]
            {
                new CategoryAttributeDateFieldVM(new CategoryAttributeDateField("DateField", null)),
                new CategoryAttributeIntegerFieldVM(new CategoryAttributeIntegerField("IntField", null))
            });

        CopyToChildrenCommand = new RelayCommand<CategoryAttributeVM>(DoCopyToChildrenCommand);
    }

    public IEnumerable<CategoryAttributeVM> CategoryAttributeVMs { get; private set; }
    private IEnumerable<CategoryAttributeVM> CategoryAttributeVM2s { get; private set; }

    // This an MVVM Light RelayCommand
    public RelayCommand<CategoryAttributeVM> CopyToChildrenCommand { get; private set; }

    private void DoCopyToChildrenCommand(CategoryAttributeVM ca)
    {
        foreach (var item in CategoryAttributeVM2s)
        {
            // How to copy the ca.Value to item.Value where ca and item are of the same type?
            if (item.GetType() == ca.GetType())
                item.Value = ca.Value;  // !! No worky because item.Value refers to the CategoryAttributeVM.Value, which is a readonly Object property
        }
    }
}

How can I get the objects from CategoryAttributeVM2s that match the type of ‘ca’?
I think I am missing something elementary, but cannot see it.

The generics approach would be nice but AFAIK the RelayCommand cannot be made generic in that way:

// This would be great
private void DoCopyToChildrenCommand<T, U>(T ca) where T : CategoryAttributeVM<U>
{
    foreach (var item in CategoryAttributeVM2s.OfType<T>())
        item.Value = ca.Value;  // item.Value is type U, which is perfect
}

// But this can't be, obviously
public RelayCommand<CategoryAttributeVM<U>> CopyToChildrenCommand<U> { get; private set; }

Any ideas?

Answer 1

In the end, I added an overload of an abstract method on CategoryAttributeVM and CategoryAttributeVM<T>:

public abstract class CategoryAttributeVM 
{ 
    <...>
    public abstract void UpdateValueFrom(CategoryAttributeVM sourceCategoryAttributeVM);
} 

public abstract class CategoryAttributeVM<T> : CategoryAttributeVM 
{
    <...>
    public override void UpdateValueFrom(CategoryAttributeVM sourceCategoryAttributeVM)
    {
        if (GetType() == sourceCategoryAttributeVM.GetType())
            Value = (T)sourceCategoryAttributeVM.Value;
    }
}

I could then use this in DoCopyToChildrenCommand:

private void DoCopyToChildrenCommand(CategoryAttributeVM ca)
{
    foreach (var attribute in CategoryAttributeVM2s)
        attribute.UpdateValueFrom(ca);
}