Blazor Startup Error: System.Threading.SynchronizationLockException: Cannot wait on monitors on this runtime

Question

I am trying to call an api during the blazor(client side) startup to load language translations into the ILocalizer.

At the point I try and get the .Result from the get request blazor throws the error in the title.

This can replicated by calling this method in the program.cs

  private static void CalApi()
    {
        try
        {
            HttpClient httpClient = new HttpClient();
            httpClient.BaseAddress = new Uri(@"https://dummy.restapiexample.com/api/v1/employees");
            string path = "ididcontent.json";
            string response = httpClient.GetStringAsync(path)?.Result;
            Console.WriteLine(response);
        }
        catch(Exception ex)
        {
            Console.WriteLine("Error getting api response: " + ex);
        }

    }

Henk Holterman · Accepted Answer · 2020-08-02 07:58:49Z

14

Avoid .Result, it can easily deadlock. You get this error because the mechanism is not (cannot be) supported on single-threaded webassembly. I would consider it a feature. If it could wait on a Monitor it would freeze.

private static async Task CalApi()
{
   ... 
   string response = await httpClient.GetStringAsync(path); 
   ...
}

All events and lifecycle method overrides can be async Task in Blazor, so you should be able to fit this in.

edited Aug 2, 2020 at 7:58

answered Aug 2, 2020 at 5:42

Henk Holterman

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2 Comments

agua from mars Over a year ago

or in the DI setup

Evgeny Gorbovoy Over a year ago

DI container requires sync resolution of the services. If any needs async operation, than it's impossible to do in blazor

Brian Parker · Accepted Answer · 2020-08-02 11:57:41Z

4

In Program.cs

 public static async Task Main(string[] args)
 {

    ......

    builder.Services.AddSingleton<SomeService>();

    var host = builder.Build();       

    ...

call your code here but use await

    var httpClient = host.Services.GetRequiredService<HttpClient>();
    string response = await httpClient.GetStringAsync(path);
    ...

    var someService = host.Services.GetRequiredService<SomeService>();
    someService.SomeProperty = response;

    await host.RunAsync();

edited Aug 2, 2020 at 11:57

answered Aug 2, 2020 at 10:57

Brian Parker

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1 Comment

JKerny Over a year ago

good suggestion. I think Ilocalizer isn't async but yeah I think you're right I might have to right my own

Didier Restrepo · Accepted Answer · 2020-11-25 15:35:24Z

1

This is a example best:

var client= new ProductServiceGrpc.ProductServiceGrpcClient(Channel);
            category =  (await client.GetCategoryAsync(new GetProductRequest() {Id = id})).Category;

answered Nov 25, 2020 at 15:35

Didier Restrepo

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