# Component Association

### Parent Component

For `.razor` file component, you should create cascading component manually

{% code title="List.razor" %}

```cshtml
@inherits BlazorComponentBase

<CascadingValue Value="this">
    <ul @attributes="AdditionalAttributes">@ChildContent</ul>
</CascadingValue>
```

{% endcode %}

For component class, just define `ParentComponent` attribute

```csharp
[ParentComponent]
public class List : BlazorComponentBase
{
}
```

### Child Component

Define `ChildComponent` attribute for child component

{% code title="ListItem.razor" %}

```cshtml
@inherits BlazorComponentBase
@attribute [ChildComponent(typeof(List))]

<!--For .NET 7 can use generic attribute-->
@attribute [ChildComponent<List>]

<li @attributes="@AdditionalAttributes">
    @ChildContent
</li>

@code{
    [CascadingParameter]public List CascadedList { get; set; }
    
    protected override void BuildCssClass(ICssClassBuilder builder)
    {
        if(CascadedList.Active)
        {
            //..
        }
    }
}
```

{% endcode %}

{% code title="ListItem.cs" %}

```csharp
[ChildComponent(typeof(List))]
//[ChidlComponent<List>] // for .net7
public class ListItem : BlazorComponentBase
{
    [CascadingParameter]public List CascadedList { get; set; }
    
    protected override void BuildCssClass(ICssClassBuilder builder)
    {
        if(CascadedList.Active)
        {
            //..
        }
    }
}
```

{% endcode %}

So `ListItem` component must be inside of `List` component, or an exception will be thrown if `ListItem` dose not inside of `List` component

```cshtml
<List>
    <ListItem>...</ListItem>
</List>

<!--throw exception-->
<ListItem />
```

### Optional

Set `true` ignore the exception if not in parent component

{% code title="ListItem.razor" %}

```cshtml
@inherits BlazorComponentBase
@attribute [ChildComponent(typeof(List))]

<!--For .NET 7 can use generic attribute-->
@attribute [ChildComponent<List>]

<li @attributes="@AdditionalAttributes">
    @ChildContent
</li>

@code{
    [CascadingParameter]public List CascadedList { get; set; }
    
    protected override void BuildCssClass(ICssClassBuilder builder)
    {
        if(CascadedList.Active)
        {
            //..
        }
    }
}
```

{% endcode %}

{% code title="ListItem.cs" %}

```csharp
[ChildComponent(typeof(List), Optional = true)]
//[ChidlComponent<List>(Optional = true)] // for .net7
public class ListItem : BlazorComponentBase
{
    //This instance may be null
    [CascadingParameter]public List? CascadedList { get; set; }
    
    protected override void BuildCssClass(ICssClassBuilder builder)
    {
        if(CascadedList is not null && CascadedList.Active)
        {
            //..
        }
    }
}
```

{% endcode %}

```cshtml
<!--never throw exception-->
<ListItem />
```


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