App Services – My Point of View

Show/Hide element in Blazor WebAssembly

Blazor WebAssembly doesn’t allow direct manipulation of DOM. Here is how to show/hide DOM element without using Javascript Interop;

The hidden html attribute also works to hide an element.

<p hidden>This paragraph should be hidden.</p>

To bind to Model:

 <p hidden="@HideLabel">I am Hidden When HideLabel == true</p>

 <p hidden="@(!HideLabel)">I am Hidden when Hidelabel == false</p>
    
 <button @onclick="@Toggle">Show/Hide</button>

 @code {
      private bool HideLabel { get; set; } = false;
      private void Toggle()
      {
         HideLabel =   !HideLabel;
      }      
 }

Edit: You can also use a CSS class to hide/show an element:

<div class="font-italic @(HideLabel ? "d-none" : "d-show")">
   I am Hidden When HideLabel == true
</div>

Reference

https://stackoverflow.com/questions/63693734/how-to-show-hide-an-element-in-real-time-blazor

Convert Enum to List in C#

We can use LINQ for this;

public class EnumModel
{
    public int Value { get; set; }
    public string Name { get; set; }
}

public enum MyEnum
{
    Name1=1,
    Name2=2,
    Name3=3
}

public class Test
{
        List<EnumModel> enums = ((MyEnum[])Enum.GetValues(typeof(MyEnum))).Select(c => new EnumModel() { Value = (int)c, Name = c.ToString() }).ToList();

        // A list of Names only, does away with the need of EnumModel 
        List<string> MyNames = ((MyEnum[])Enum.GetValues(typeof(MyEnum))).Select(c => c.ToString()).ToList();

        // A list of Values only, does away with the need of EnumModel 
        List<int> myValues = ((MyEnum[])Enum.GetValues(typeof(MyEnum))).Select(c => (int)c).ToList();

        // A dictionnary of <string,int>
        Dictionary<string,int> myDic = ((MyEnum[])Enum.GetValues(typeof(MyEnum))).ToDictionary(k => k.ToString(), v => (int)v);
}

Reference

https://stackoverflow.com/questions/1167361/how-do-i-convert-an-enum-to-a-list-in-c

StateHasChanged() vs InvokeAsync(StateHasChanged) in Blazor WebAssembly

I have tried calling StateHasChanged() – instead of InvokeAsync(StateHasChanged) – in a Timer’s Elapsed event, and it works as expected

That must have been on WebAssembly. When you try that on Blazor Serverside I would expect an exception. StateHasChanged() checks if it runs on the right thread.

The core issue is that the rendering and calling StateHasChanged both have to happen on the main (UI) thread. Actually that is “on the SynchronizationContext” but for all intents and purposes you can think of it as being a single-thread, just as in WinForms, WPF and other GUIs). The virtual DOM is not thread-safe.

The main Blazor life-cycle events (OnInit, AfterRender, ButtonClick) are all executed on that special thread so in the rare case that you need StateHasChanged() there it can be called without InvokeAsync().

A Timer is different, it is an “external event” so you can’t be sure it will execute on the correct thread. InvokeAsync() delegates the work to Blazor’s SynchronizationContext that will ensure it does run on the main thread.

But Blazor WebAssembly only has 1 thread so for the time being external events always run on the main thread too. That means that when you get this Invoke pattern wrong you won’t notice anything. Until one day, when Blazor Wasm finally gets real threads, your code will fail. As is the case with your Timer experiment.

What is “Blazor’s synchronization context”?

In .net a synchronization context determines what happens with (after) await. Different platforms have different settings, the Blazor synccontext is a lot like that of WinForms and WPF. Mainly, the default is .ConfigureAwait(true): resume on the same thread/context.

I sometimes see .ConfigureAwait(false) in toplevel Blazor Wasm code. That too will blow up when we get real threads there. It is fine to use in services called from Blazor, but not for the toplevel methods.

And finally, await InvokeAsync(StateHasChanged) or await InvokeAsync(() => StateHasChanged()) is just about lambda’s in C#, nothing to do with Blazor. The first short form is a little more efficient.

I also sometimes see InvokeAsync() called without await

That will work. It probably is better than the other option: making the calling method (like the Timer’s OnTick) an async void. So do use this from a synchronous code path.

Reference

https://stackoverflow.com/questions/65230621/statehaschanged-vs-invokeasyncstatehaschanged-in-blazor

400 vs 422 response in post action

There are three possible types of client errors on API calls that receive request bodies:

Sending invalid JSON will result in a 400 Bad Request response:

HTTP/1.1 400 Bad Request
Content-Length: 35
{"message":"Problems parsing JSON"}

Sending the wrong type of JSON values will result in a 400 Bad Request response:

HTTP/1.1 400 Bad Request
Content-Length: 40

{"message":"Body should be a JSON object"}

Sending invalid fields will result in a 422 Unprocessable Entity response:

HTTP/1.1 422 Unprocessable Entity
Content-Length: 149

{
  "message": "Validation Failed",
  "errors": [
    {
      "resource": "Issue",
      "field": "title",
      "code": "missing_field"
    }
  ]
}

Reference

https://stackoverflow.com/questions/16133923/400-vs-422-response-to-post-of-data

Blazor WebAssembly and Antiforgery token

EditForm comes with built-in anti-forgery token support. Blazor automatically secures the EditForm instances, saving you the hassle of explicitly handling CSRF protection.

Blazor WebAssembly apps run entirely in the browser and do not have a server-side processing pipeline where you would typically configure a middleware such as app.UseAntiforgery(). If your Blazor WebAssembly app interacts with server-side APIs, you should manage anti-forgery at the API level. However, if you already use token-based authentication to secure communication, anti-forgery tokens are generally not necessary. Token-based authentication, by its nature, mitigates the risks associated with CSRF, making additional anti-forgery tokens redundant.

Reference

https://learn.microsoft.com/en-us/xandr/digital-platform-api/token-based-api-authentication