BuildRenderTree is not called in a razor page that inherits of my custom component

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05-10-2024

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Why Your Razor Page Isn't Building the RenderTree: A Deep Dive into Custom Component Inheritance

Problem: You've built a custom component in your Blazor application, and you're trying to inherit it in a Razor page. But, you're encountering a frustrating issue: the BuildRenderTree method of your custom component isn't getting called. This means your component's UI isn't rendering as expected.

Rephrased: Imagine you've built a beautiful custom house with a fantastic kitchen. You want to use this house as a blueprint for a new apartment building. But, when you build the apartments, the kitchen is nowhere to be seen! That's essentially what's happening here – your Razor page isn't recognizing the BuildRenderTree method of your custom component, so the UI elements aren't being displayed.

Scenario and Code:

Let's illustrate this with a simple example. Suppose we have a custom component called MyCustomComponent:

using Microsoft.AspNetCore.Components;

namespace MyBlazorApp.Components
{
    public partial class MyCustomComponent : ComponentBase
    {
        [Parameter]
        public string Message { get; set; }

        protected override void BuildRenderTree(Microsoft.AspNetCore.Components.Rendering.RenderTreeBuilder builder)
        {
            builder.OpenElement(0, "div");
            builder.AddAttribute(1, "class", "custom-component");
            builder.AddContent(2, {{content}}quot;Hello from the component: {Message}");
            builder.CloseElement();
        }
    }
}

And a Razor page that inherits from it:

@inherits MyCustomComponent

@code {
    public string Message = "World";
}

You'd expect to see "Hello from the component: World" displayed in the browser. However, if the BuildRenderTree method isn't getting called, you'll see nothing.

Analysis and Insights:

The root of the problem lies in the way Blazor handles inheritance and component rendering. When you inherit a component, Blazor doesn't automatically call the base component's BuildRenderTree method. Instead, it renders the inherited component from scratch, effectively ignoring the base component's structure.

Solutions and Examples:

Here are two ways to resolve this issue and ensure your custom component is correctly rendered:

1. Use the @layout directive:

   Instead of directly inheriting the custom component, use the @layout directive in your Razor page:

   @page "/mypage"
   @layout MyCustomComponent
   
   @code {
       public string Message = "World";
   }
   

   The @layout directive instructs Blazor to render the content of your Razor page within the structure of the custom component. This ensures that the BuildRenderTree method of your custom component is called and the UI elements are displayed as intended.

2. Compose the component:

   Instead of inheritance, you can achieve a similar effect by composing the custom component within your Razor page:

   @page "/mypage"
   
   @code {
       public string Message = "World";
   }
   
   <MyCustomComponent Message="@Message" />
   

   This explicitly tells Blazor to render the MyCustomComponent with the specified Message parameter. This ensures that the custom component's BuildRenderTree method is called, resulting in its UI elements being displayed.

Additional Value and Benefits:

Understanding how inheritance works in Blazor is crucial for creating reusable and modular components. By using the techniques outlined above, you can leverage your custom components effectively within your Razor pages.

References and Resources:

• Blazor Documentation: Components
• Blazor Documentation: Layouts

By applying these solutions and understanding the intricacies of component inheritance, you can build sophisticated Blazor applications with ease.