使用将接口而不是具体类传递给@ChildContent 的 [CascadingParameter]<cascadingvalue> 在 blazor</cascadingvalue>

Question

I have Item implementing IItem , and ItemContainer implementing IItemContainer .

In ItemContainer.razor I have the following, passing itself to all its @ChildContent :

<CascadingValue Value="this">
    @ChildContent
</CascadingValue>

@code{
  List<IItem> Items;
  void AddItem(IItem item) => Items.Add(item);
  .
  .
  .
}

And in the Item class I have the following:

[CascadingParameter] public IItemContainer ItemContainer {get;set;}
protected override void OnInitialized()
{
  ItemContainer.AddItem(this);
}
RenderFragment RenderStuff => Do some rendering stuff with ItemContainer
.
.
.

Say that I have the following:

<ItemContainer>
  <Item>
  <Item>
  <Item>
</ItemContainer>

I expect that by the time the first <Item> calls its OnInitialized , the <ItemContainer> would already have passed itself into [CascadingParameter] public IItemContainer ItemContainer {get;set;} , so [CascadingParameter] public IItemContainer ItemContainer {get;set;} will not be null, and ItemContainer.AddItem(this); will be successfully called and add the <Item> to the <ItemContainer> 's List<IItem> Items .

This is not an issue if I use concrete classes, ie ItemContainer instead of IItemContainer for <CascadingValue Value="this"> .

However, when the this of <CascadingValue Value="this"> is an interface and not an concrete class, casting does not automatically happen, and null exception will be thrown (ItemContainer not passed into the cascading param).

And when I try to cast it using <CascadingValue TValue="IItemContainer" Value="(IItemContainer)this"> , exception will be thrown.

I want to pass interfaces instead of concrete classes into cascading params. Is there a way to make this work?

Answer 1

And when I try to cast it using

<CascadingValue TValue="IItemContainer" 
  Value="(IItemContainer)this">

exception will be thrown.

You don't do that like this. You still have to pass this

Here's the way to do that:

ItemContainer.razor

<CascadingValue Value="this">
    @ChildContent
</CascadingValue>

@code{
  List<IItem> Items;
  void AddItem(IItem item) => Items.Add(item);
  .
  .
  .
}

Item.razor

@implements IItem
 
[CascadingParameter] public IItemContainer ItemContainer {get;set;}

protected override void OnInitialized()
{
    // Note: The first ItemContainer (left to right) is the class name,
    // the second is the name of the cascading parameter property. If the compiler
    // fail to recognize that (I guess she won't), change this property to 
    // ContainerItemSet, as for instance. Otherwise, it should work perfectly 
    // fine. 
    ((ItemContainer)(object)(ItemContainer)).AddItem(this);
}