Here's a modified version of your code that does what you want it to:

@page "/"
<label>Count:</label>
<button @onclick=Increment>@count times!</button>
<br>
A:
<input @oninput=OnChange value="@count">
<br>
B:
<input @bind-value=count @bind-value:event="oninput">

@code {
    int count = 1;

    void Increment() => count++;

    async Task OnChange(ChangeEventArgs e)
    {
        var oldvalue = count;
        var isNewValue = int.TryParse(e.Value?.ToString(), out var v);
        if (isNewValue)
            count = v;
        else
        {
            count = 0;
            // this one line may precipitate a few commments!
            await Task.Yield();
            count = oldvalue;
        }

    }
}

So "What's going on?"

Firstly razor code is pre-compiled into C# classes, so what you see is not what actually gets run as code. I won't go into that here, there's plenty of articles online.

value="@count" is a one way binding, and is passed as a string.

You may change the actual value in the input on screen, but in the Blazor component the value is still the old value. There's been no callback to tell it otherwise.

When you type 22x after 22, OnChange doesn't update count. As far as the Renderer is concerned it hasn't changed so it don't need to update that bit of the the DOM. We have a mismatch between the Renderer DOM and the actual DOM!

OnChange changes to async Task and it now:

• Gets a copy of the old value
• If the new value is a number updates count.
• If it's not a number

1. Sets count to another value - in this case zero.
2. Yields. The Blazor Component Event handler calls StateHasChanged and yields. This gives the Renderer thread time to service it's queue and re-render. The input in momentarily zero.
3. Set count to the old value.
4. Returns Task complete. The Blazor Component Event handler runs to completion calling StateHasChanged a second time. The Renderer updates the display value.

Update on why Task.Yield is used

The basic Blazor Component event handler [BCEH from this point] looks like this:

var task = InvokeAsync(EventMethod);
StateHasChanged();
if (!task.IsCompleted)
{
    await task;
    StateHasChanged();
}

Put OnChange into this context.

var task = InvokeAsync(EventMethod) runs OnChange. Which starts to run synchronously.

If isNewValue is false it's sets count to 0 and then yields through Task.Yield passing an incomplete Task back to BCEH. This can then progress and runs StateHasChanged which queues a render fragment onto the Renderer's queue. Note it doesn't actually render the component, just queues the render fragment. At this point BCEH is hogging the thread so the Renderer can't actually service it's queue. It then checks task to see if it's completed.

If it's complete BCEH completes, the Renderer gets some thread time and renders the component.

If it's still running - it will be as we've kicked it to the back of the thread queue with Task.Yield - BCEH awaits it and yields. The Renderer gets some thread time and renders the component. OnChange then completes, BCEH gets a completed Task, stacks another render on the Render's queue with a call to StateHasChanged and completes. The Renderer, now with thread time services it's queue and renders the component a second time.

Note some people prefer to use Task.Delay(1), because there's some discussion on exactly how Task.Yield works!

This was a really interesting question. I've never used Blazor before, but I had an idea about what might help here, albeit this is a hacky answer too.

I noticed if you changed the element to bind to the count variable, it would update the value when the control lost focus. So I added some code to swap focusing elements. This seems to allow typing non-numeric characters without changing the input field, which is what I think is desired here.

Obviously, not a fantastic solution but thought I'd offer it up in case it helps.

<label>Count:</label>
<button @onclick=Increment>@count times!</button><br>
A: <input @oninput=OnChange @bind-value=count @ref="textInput1"><br>
B: <input @bind-value=count @bind-value:event="oninput" @ref="textInput2">

@code {
    ElementReference textInput1;
    ElementReference textInput2;
    int count = 1;

    void Increment() => count++;

    void OnChange(ChangeEventArgs e)
    {
        var userValue = e.Value?.ToString(); 
        if (int.TryParse(userValue, out var v))
        {
            count = v;
        }
        else 
        {
            if (String.IsNullOrWhiteSpace(userValue))
                count = 0;
                
            e.Value = count.ToString();

            textInput2.FocusAsync();        
            textInput1.FocusAsync();
        }
    }
}

So, basically I need to re-render A to undo invalid user input, even thought the state hasn't changed. How can I do that without the bind-xxx magic?

You can force recreating and rerendering of any element/component by changing value in @key directive:

<input @oninput=OnChange value="@count" @key="version" />

void OnChange(ChangeEventArgs e)
{
    var userValue = e.Value?.ToString(); 
    if (int.TryParse(userValue, out var v))
    {
        count = v;
    }
    else 
    {
        version++;
        if (String.IsNullOrWhiteSpace(userValue))
        {
            count = 0;
        }         
   }
}

Notice, that it will rerender the entire element (and it's subtree), not just the attribute.

The problem with your code is, that the BuildRendrerTree method of your component generates exactly the same RenderTree, so the diffing algorithm doesn't find anything to update in the actual dom.

So why the @bind directive works?

Notice the generated BuildRenderTree code:

//A
__builder.OpenElement(6, "input");
__builder.AddAttribute(7, "oninput", Microsoft.AspNetCore.Components.EventCallback.Factory.Create<Microsoft.AspNetCore.Components.ChangeEventArgs>(this, OnChange));
__builder.AddAttribute(8, "value", count);
__builder.CloseElement();

//B
__builder.AddMarkupContent(9, "<br>\r\nB: ");
__builder.OpenElement(10, "input");
__builder.AddAttribute(11, "value", Microsoft.AspNetCore.Components.BindConverter.FormatValue(count));
__builder.AddAttribute(12, "oninput", Microsoft.AspNetCore.Components.EventCallback.Factory.CreateBinder(this, __value => count = __value, count));
__builder.SetUpdatesAttributeName("value");
__builder.CloseElement();

The trick is, that @bind directive adds:

__builder.SetUpdatesAttributeName("value");

You can't do this in markup for EventCallback right now, but there is an open issue for it: https://github.com/dotnet/aspnetcore/issues/17281

However you still can create a Component or RenderFragment and write the __builder code manually.

Below is what I've come up with (try online). I wish ChangeEventArgs.Value worked both ways, but it doesn't. Without it, I can only think of this JS.InvokeVoidAsync hack:

@inject IJSRuntime JS

<label>Count:</label>
<button @onclick=Increment>@count times!</button>
<br>
A:
<input @oninput=OnChange value="@count" id="@id">
<br>
B:
<input @bind-value=count @bind-value:event="oninput">

@code {
    int count = 1;
    string id = Guid.NewGuid().ToString("N");

    void Increment() => count++;

    async Task OnChange(ChangeEventArgs e)
    {
        var userValue = e.Value?.ToString(); 
        if (int.TryParse(userValue, out var v))
        {
            count = v;
        }
        else 
        {
            if (String.IsNullOrWhiteSpace(userValue))
            {
                count = 0;
            }

            // this doesn't work
            e.Value = count.ToString();

            // this doesn't work either (no rererendering):
            StateHasChanged();           

            // using JS interop as a workaround 
            await JS.InvokeVoidAsync("eval",
                $"document.getElementById('{id}').value = Number('{count}')");
        }
    }
}

To be clear, I realize this is a horrible hack (last but not least because it uses eval); I could possibly improve it by using ElementReference and an isolated JS import, but all of that wouldn't be necessary if e.Value = count just worked, like it does with Svelte. I've raised this as a Blazor issue in ASP.NET repo, hopefully it might get some attention.

From looking at the code, it seems you wanna sanitize user input? E.g. can enter only numbers, date formats etc... I agree it's kinda hard if you wanna use the event handlers manually in this regard at the moment, but it's still possible to validate input using expanded properties and Blazor's binding system.

What you want would look like this: (try it here)

<label>Count: @Count</label>
<button @onclick=Increment>@Count times!</button><br>
<input @bind-value=Count @bind-value:event="oninput">

@code {
    int _count = 1;
    public string Count 
    {
        get => _count.ToString();
        set 
        {
            if(int.TryParse(value, out var val)) {
                _count = val;
            }
            else {
                if(string.IsNullOrEmpty(value)) {
                    _count = 0;
                }
            }
        }
    }

    void Increment() => _count++;
}

You can use @on{DOM EVENT}:preventDefault to prevent the default action for an event. For more information look at Microsoft docs: https://docs.microsoft.com/en-us/aspnet/core/blazor/components/event-handling?view=aspnetcore-6.0#prevent-default-actions

UPDATE

An example of using preventDefault

<label>Count:</label>
<button @onclick=Increment>@count times!</button><br />
<br>
A: <input value="@count" @onkeydown=KeyHandler @onkeydown:preventDefault><br>
B: <input @bind-value=count @bind-value:event="oninput">

@code {
    int count = 1;
    string input = "";

    void Increment() => count++;

    private void KeyHandler(KeyboardEventArgs e)
    {
        //Define your pattern
        var pattern = "abcdefghijklmnopqrstuvwxyz";
        if (!pattern.Contains(e.Key) && e.Key != "Backspace")
        {
            //This is just a sample and needs exception handling
            input = input + e.Key;
            count = int.Parse(input);
        }
        if (e.Key == "Backspace")
        {
            input = input.Remove(input.Length - 1);
            count = int.Parse(input);
        }
    }
}

You will have to use @on{DOM EVENT}-preventDefault.

It is coming from the javascript world and it prevents the default behavior of the button.

In blazor and razor, validation or DDL trigger postback, which means the request is processed by the server and re-renders.

When doing this, you need to make sure your event does not 'bubble' as you are preventing from the event to perform postback.

If you find your event bubbling, meaning element event going up the elements it is nested in, please use:

stopPropagation="true";

for more info:

Suppressing events in blazor