Async calls inside constructor

Question

Async calls inside constructor

ANB 181

I have a service and inside its constructor, I make 2 asynchronous calls.
However I am getting the message "the await operator can only be used within an async method. Consider marking this method with the async modifier and changing its return type to Task"

public MyService(HttpClient httpClient) {
_httpClient = httpClient;
_httpClient.DefaultRequestHeaders.Add("param1", await _localStorageService.GetItemAsync<string>("param1");
_httpClient.DefaultRequestHeaders.Add("param2", await _localStorageService.GetItemAsync<string>("param2");
}

This methods come from a nuget package Blazored.Localstorage.
So how can I make this work ?

Thanks

AgaveJoe 30,126 Reputation points

2022-09-26T18:06:12.267+00:00

You have design issues if you need make asynchronous calls from a class constructor. Typically, the HttpClient is configured when the application starts similar to a service. Perhaps use a factory.

https://learn.microsoft.com/en-us/dotnet/architecture/microservices/implement-resilient-applications/use-httpclientfactory-to-implement-resilient-http-requests

If you have parameters stored in local storage then fetch the parameters when making the call.
ANB 181 Reputation points

2022-09-26T18:55:21.317+00:00

The problem here is I need to get some data from the cookies and then using them on my constructor, adding their values to my HttpClient.DefaultHeaders.
So what would be the right way ?
AgaveJoe 30,126 Reputation points

2022-09-26T19:42:40.15+00:00
You can try adding .Result to the async method result.

public MyService(HttpClient httpClient) { _httpClient = httpClient; _httpClient.DefaultRequestHeaders.Add("param1", _localStorageService.GetItemAsync<string>("param1").Result); _httpClient.DefaultRequestHeaders.Add("param2", _localStorageService.GetItemAsync<string>("param2").Result); }

There could be a better solution if you explain the high level problem you are trying to solve. Also, it is a bit confusing that you're calling local storage unless this is a Blazor WASM application. Your follow up post mentions cookies which is a bit more confusing if the parameters contain sensitive data. What kind of application are you building?
ANB 181 Reputation points

2022-09-26T20:00:19.603+00:00

It's a blazor server app.
But the way people did was when the user logs in, it will create some cookies with user params.
So later, it will call API's passing these user params.
AgaveJoe 30,126 Reputation points

2022-09-26T20:49:06.49+00:00

But the way people did was when the user logs in, it will create some cookies with user params. So later, it will call API's passing these user params.

Yeah, this is standard concept that has been solved. Middleware reads the cookie and populates whatever it is your trying to populate. Usually it is the user's identity from an authentication cookie. If you doing something with a JWT and I recall correctly the standard Blazor template has this baked in.

I think you might be reinventing the wheel.

1 answer

Your answer

AgaveJoe 30,126 Reputation points

2022-09-26T18:06:12.267+00:00

You have design issues if you need make asynchronous calls from a class constructor. Typically, the HttpClient is configured when the application starts similar to a service. Perhaps use a factory.

https://learn.microsoft.com/en-us/dotnet/architecture/microservices/implement-resilient-applications/use-httpclientfactory-to-implement-resilient-http-requests

If you have parameters stored in local storage then fetch the parameters when making the call.
ANB 181 Reputation points

2022-09-26T18:55:21.317+00:00

The problem here is I need to get some data from the cookies and then using them on my constructor, adding their values to my HttpClient.DefaultHeaders.
So what would be the right way ?
AgaveJoe 30,126 Reputation points

2022-09-26T19:42:40.15+00:00

You can try adding .Result to the async method result.

public MyService(HttpClient httpClient) { _httpClient = httpClient; _httpClient.DefaultRequestHeaders.Add("param1", _localStorageService.GetItemAsync<string>("param1").Result); _httpClient.DefaultRequestHeaders.Add("param2", _localStorageService.GetItemAsync<string>("param2").Result); }

There could be a better solution if you explain the high level problem you are trying to solve. Also, it is a bit confusing that you're calling local storage unless this is a Blazor WASM application. Your follow up post mentions cookies which is a bit more confusing if the parameters contain sensitive data. What kind of application are you building?
ANB 181 Reputation points

2022-09-26T20:00:19.603+00:00

It's a blazor server app.
But the way people did was when the user logs in, it will create some cookies with user params.
So later, it will call API's passing these user params.
AgaveJoe 30,126 Reputation points

2022-09-26T20:49:06.49+00:00

But the way people did was when the user logs in, it will create some cookies with user params. So later, it will call API's passing these user params.

Yeah, this is standard concept that has been solved. Middleware reads the cookie and populates whatever it is your trying to populate. Usually it is the user's identity from an authentication cookie. If you doing something with a JWT and I recall correctly the standard Blazor template has this baked in.

I think you might be reinventing the wheel.

Answer 1

Bruce (SqlWork.com) 77,686 Volunteer Moderator

constructor method can not be async. you can start an async/task process, but you can not await it.

as the service calls should all be async. there is an easy fix. provide an async GetHttpclient() routine use instead of using _httpClient directly

private bool GetHttpClientInit = false;  
private async Task<HttpClient> GetHttpClient()  
{  
   if (! GetHttpClientInit)  
   {  
         _httpClient.DefaultRequestHeaders.Add("param1", await _localStorageService.GetItemAsync<string>("param1");  
         _httpClient.DefaultRequestHeaders.Add("param2", await _localStorageService.GetItemAsync<string>("param2");  
         GetHttpClientInit = true;  
   }  
   return _httpClient;  
}

ANB 181 Reputation points

2022-09-26T20:40:39.837+00:00
But then how would it work?
For each method I would have to call this GetHttpClient again ?

eg:
public async Task<int> SaveInfo()
{
await GetHttpClient();
...
var response = await _httpClient.SendAsync(httpRequestMessage);

if (response.StatusCode == System.Net.HttpStatusCode.OK) ...
Jason Wong 0 Reputation points

2023-02-15T01:47:23.3133333+00:00

The problem with doing that is that it is possible for the execution to proceed from that and the next piece of code that is called will try to use _httpClient before the awaited code from before actually runs. Javascript just doesn't support execution context yielding like an OS does because javascript just sucks to begin with.

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Async calls inside constructor

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