Blazor BuildRenderTree

In Blazor, all components and pages end up as a generated class by the compiler. The generated class inherits from the BlazorComponent base class and uses the BuildRenderTree function to generate the contents. Instead of depending on this class generation process, you can also write your own C# class directly to generate the contents.

Basically, your class need to inherit from BlazorComponent and override the BuildRenderTree function to generate dynamic content.

Pages/ClassOnlyComponent.cs


using Microsoft.AspNetCore.Blazor.Components;
using Microsoft.AspNetCore.Blazor.RenderTree;
using Microsoft.AspNetCore.Blazor.Layouts;
using Blazor_Dashboard.Shared;

namespace BlazorDashboard.Pages
{
    [Route("/classrender")]
    [Layout(typeof(MainLayout))]
    public class ClassOnlyComponent :BlazorComponent
{
    protected override void BuildRenderTree(RenderTreeBuilder builder)
    {
        var seq = 0;
        builder.OpenElement(seq, "h3");
        builder.AddContent(++seq, "BuildRenderTree");
        builder.CloseElement();
    }
}
}

A more interesting example below shows you how to a draw a Rectangle in SVG (Scalable Vector Graphics).

Pages/ClassOnlyComponent.cs


using Microsoft.AspNetCore.Blazor.Components;
using Microsoft.AspNetCore.Blazor.RenderTree;
using Microsoft.AspNetCore.Blazor.Layouts;
using Blazor_Dashboard.Shared;

namespace BlazorDashboard.Pages
{
    [Route("/classrender")]
    [Layout(typeof(MainLayout))]
    public class ClassOnlyComponent :BlazorComponent
{
    protected override void BuildRenderTree(RenderTreeBuilder builder)
    {
            var seq = 0;

            builder.OpenElement(seq, "figure");
            builder.OpenElement(++seq, "div");
            builder.AddAttribute(++seq, "class", "doughnut-main");

            builder.OpenElement(++seq, "svg");
            builder.AddAttribute(++seq, "width", "100%");
            builder.AddAttribute(++seq, "height", "100%");
            builder.AddAttribute(++seq, "viewBox", "0 0 42 42");

            builder.OpenElement(++seq, "rect");
            builder.AddAttribute(++seq, "width", "100%");
            builder.AddAttribute(++seq, "height", "100%");
            builder.AddAttribute(++seq, "fill", "cyan");
            builder.CloseElement();
            builder.CloseElement();
            builder.CloseElement();
            builder.CloseElement();
    }
}
}

Besides adding child elements or attributes, you can also bind to variables or functions. The following shows you how to bind a variable and function in BuildRenderTree. The behavior is such that when the button is clicked, the title is updated.

When the button is clicked, title changes


using Microsoft.AspNetCore.Blazor.Components;
using Microsoft.AspNetCore.Blazor.RenderTree;
using Microsoft.AspNetCore.Blazor.Layouts;
using Blazor_Dashboard.Shared;
using System.Threading.Tasks;
using System;
using Microsoft.AspNetCore.Blazor;

namespace BlazorDashboard.Pages
{
    [Route("/classrender")]
    [Layout(typeof(MainLayout))]
    public class ClassOnlyComponent : BlazorComponent
    {
        protected override void BuildRenderTree(RenderTreeBuilder builder)
        {
            Action act1 = new Action((s) =>
            {
                onclick(s);
            });
            var seq = 0;
            builder.OpenElement(seq, "h3");
            builder.AddContent(++seq, title);
            builder.CloseElement();
            builder.OpenElement(++seq, "button");
            builder.AddAttribute(++seq, "onclick", onclick);
            builder.AddContent(++seq, "Click");
            builder.CloseElement();
            base.BuildRenderTree(builder);
        }        
             
        protected string title = "BuildRenderTree Action";
        public void onclick(UIMouseEventArgs e)
        {
            System.Diagnostics.Debug.WriteLine("Hello");
            title = "Hello OnClick";

        }
        
    }
}