ASP.NET Core Blazor component rendering

Components must render when they're first added to the component hierarchy by a parent component. This is the only time that a component must render. Components may render at other times according to their own logic and conventions.

Rendering conventions for ComponentBase

By default, Razor components inherit from the ComponentBase base class, which contains logic to trigger rerendering at the following times:

Components inherited from ComponentBase skip rerenders due to parameter updates if either of the following are true:

  • All of the parameter values are of known immutable primitive types, such as int, string, DateTime, and haven't changed since the previous set of parameters were set.
  • The component's ShouldRender method returns false.

Control the rendering flow

In most cases, ComponentBase conventions result in the correct subset of component rerenders after an event occurs. Developers aren't usually required to provide manual logic to tell the framework which components to rerender and when to rerender them. The overall effect of the framework's conventions is that the component receiving an event rerenders itself, which recursively triggers rerendering of descendant components whose parameter values may have changed.

For more information on the performance implications of the framework's conventions and how to optimize an app's component hierarchy for rendering, see ASP.NET Core Blazor WebAssembly performance best practices.

Suppress UI refreshing (ShouldRender)

ShouldRender is called each time a component is rendered. Override ShouldRender to manage UI refreshing. If the implementation returns true, the UI is refreshed.

Even if ShouldRender is overridden, the component is always initially rendered.

Pages/ControlRender.razor:

@page "/control-render"

<label>
    <input type="checkbox" @bind="shouldRender" />
    Should Render?
</label>

<p>Current count: @currentCount</p>

<p>
    <button @onclick="IncrementCount">Click me</button>
</p>

@code {
    private int currentCount = 0;
    private bool shouldRender = true;

    protected override bool ShouldRender()
    {
        return shouldRender;
    }

    private void IncrementCount()
    {
        currentCount++;
    }
}

For more information on performance best practices pertaining to ShouldRender, see ASP.NET Core Blazor WebAssembly performance best practices.

When to call StateHasChanged

Calling StateHasChanged allows you to trigger a render at any time. However, be careful not to call StateHasChanged unnecessarily, which is a common mistake that imposes unnecessary rendering costs.

Code shouldn't need to call StateHasChanged when:

  • Routinely handling events, whether synchronously or asynchronously, since ComponentBase triggers a render for most routine event handlers.
  • Implementing typical lifecycle logic, such as OnInitialized or OnParametersSetAsync, whether synchonrously or asynchronously, since ComponentBase triggers a render for typical lifecycle events.

However, it might make sense to call StateHasChanged in the cases described in the following sections of this article:

An asynchronous handler involves multiple asynchronous phases

Due to the way that tasks are defined in .NET, a receiver of a Task can only observe its final completion, not intermediate asynchronous states. Therefore, ComponentBase can only trigger rerendering when the Task is first returned and when the Task finally completes. The framework can't know to rerender a component at other intermediate points. If you want to rerender at intermediate points, call StateHasChanged at those points.

Consider the following CounterState1 component, which updates the count four times on each click:

  • Automatic renders occur after the first and last increments of currentCount.
  • Manual renders are triggered by calls to StateHasChanged when the framework doesn't automatically trigger rerenders at intermediate processing points where currentCount is incremented.

Pages/CounterState1.razor:

@page "/counter-state-1"

<p>
    Current count: @currentCount
</p>

<p>
    <button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
</p>

@code {
    private int currentCount = 0;

    private async Task IncrementCount()
    {
        currentCount++;
        // Renders here automatically

        await Task.Delay(1000);
        currentCount++;
        StateHasChanged();

        await Task.Delay(1000);
        currentCount++;
        StateHasChanged();

        await Task.Delay(1000);
        currentCount++;
        // Renders here automatically
    }
}

Receiving a call from something external to the Blazor rendering and event handling system

ComponentBase only knows about its own lifecycle methods and Blazor-triggered events. ComponentBase doesn't know about other events that may occur in code. For example, any C# events raised by a custom data store are unknown to Blazor. In order for such events to trigger rerendering to display updated values in the UI, call StateHasChanged.

Consider the following CounterState2 component that uses System.Timers.Timer to update a count at a regular interval and calls StateHasChanged to update the UI:

  • OnTimerCallback runs outside of any Blazor-managed rendering flow or event notification. Therefore, OnTimerCallback must call StateHasChanged because Blazor isn't aware of the changes to currentCount in the callback.
  • The component implements IDisposable, where the Timer is disposed when the framework calls the Dispose method. For more information, see ASP.NET Core Razor component lifecycle.

Because the callback is invoked outside of Blazor's synchronization context, the component must wrap the logic of OnTimerCallback in ComponentBase.InvokeAsync to move it onto the renderer's synchronization context. StateHasChanged can only be called from the renderer's synchronization context and throws an exception otherwise. This is equivalent to marshalling to the UI thread in other UI frameworks.

Pages/CounterState2.razor:

@page "/counter-state-2"
@using System.Timers
@implements IDisposable

<h1>Counter with <code>Timer</code> disposal</h1>

<p>
    Current count: @currentCount
</p>

@code {
    private int currentCount = 0;
    private Timer timer = new(1000);

    protected override void OnInitialized()
    {
        timer.Elapsed += (sender, eventArgs) => OnTimerCallback();
        timer.Start();
    }

    private void OnTimerCallback()
    {
        _ = InvokeAsync(() =>
        {
            currentCount++;
            StateHasChanged();
        });
    }

    public void Dispose() => timer.Dispose();
}

To render a component outside the subtree that's rerendered by a particular event

The UI might involve:

  1. Dispatching an event to one component.
  2. Changing some state.
  3. Rerendering a completely different component that isn't a descendant of the component receiving the event.

One way to deal with this scenario is to provide a state management class, often as a dependency injection (DI) service, injected into multiple components. When one component calls a method on the state manager, the state manager raises a C# event that's then received by an independent component.

Since these C# events are outside the Blazor rendering pipeline, call StateHasChanged on other components you wish to render in response to the state manager's events.

This is similar to the earlier case with System.Timers.Timer in the previous section. Since the execution call stack typically remains on the renderer's synchronization context, calling InvokeAsync isn't normally required. Calling InvokeAsync is only required if the logic escapes the synchronization context, such as calling ContinueWith on a Task or awaiting a Task with ConfigureAwait(false).