HttpClient not supporting PostAsJsonAsync method C#

Question

I am trying to call a web API from my web application. I am using.Net 4.5 and while writing the code I am getting the error HttpClient does not contain a definition PostAsJsonAsync method.

Below is the code:

HttpClient client = new HttpClient();
client.BaseAddress = new Uri("http://localhost:51093/");
client.DefaultRequestHeaders.Accept.Add(
   new MediaTypeWithQualityHeaderValue("application/json"));
var user = new Users();
user.AgentCode = 100;
user.Remarks = "Test";
user.CollectionDate = System.DateTime.Today;
user.RemittanceDate = System.DateTime.Today;
user.TotalAmount = 1000;
user.OrgBranchID = 101;

var response = client.PostAsJsonAsync("api/AgentCollection", user).Result;

and I am getting the error message:

Error: 'System.Net.Http.HttpClient' does not contain a definition for 'PostAsJsonAsync' and No extension method 'PostAsJsonAsync' accepting a first argument of type 'System.Net.Http.HttpClient' could be found (are you missing a using directive or an assembly reference?)

Please have a look and advice me.

Answer 1

Yes, you need to add a reference to

System.Net.Http.Formatting.dll

This can be found in the extensions assemblies area.

A good way of achieving this is by adding the NuGet package Microsoft.AspNet.WebApi.Client to your project.

Answer 2

PostAsJsonAsync is no longer in the System.Net.Http.dll (.NET 4.5.2). You can add a reference to System.Net.Http.Formatting.dll , but this actually belongs to an older version. I ran into problems with this on our TeamCity build server, these two wouldn't cooperate together.

Alternatively, you can replace PostAsJsonAsync with a PostAsync call, which is just part of new dll. Replace

var response = client.PostAsJsonAsync("api/AgentCollection", user).Result;

With:

var response = client.PostAsync("api/AgentCollection", new StringContent(
   new JavaScriptSerializer().Serialize(user), Encoding.UTF8, "application/json")).Result;

Note that JavaScriptSerializer is in the namespace: System.Web.Script.Serialization .

You will have to add an assembly reference in your csproj: System.Web.Extensions.dll

See https://code.msdn.microsoft.com/windowsapps/How-to-use-HttpClient-to-b9289836

Answer 3

The missing reference is the System.Net.Http.Formatting.dll . But the better solution is to add the NuGet package Microsoft.AspNet.WebApi.Client to ensure the version of the formatting dll worked with the .NET framework version of System.Net.Http in my project.

Answer 4

As already debatted, this method isn't available anymore since .NET 4.5.2. To expand on Jeroen K's answer you can make an extension method:

public static async Task<HttpResponseMessage> PostAsJsonAsync<TModel>(this HttpClient client, string requestUrl, TModel model)
{
    var serializer = new JavaScriptSerializer();
    var json = serializer.Serialize(model);
    var stringContent = new StringContent(json, Encoding.UTF8, "application/json");
    return await client.PostAsync(requestUrl, stringContent);
}

Now you are able to call client.PostAsJsonAsync("api/AgentCollection", user) .

Answer 5

I had this issue too on a project I'd just checked out from source control.

The symptom was the error described above and a yellow warning triangle on a reference to System.Net.Http.Formatting

To fix this, I removed the broken reference and then used NuGet to install the latest version of Microsoft.AspNet.WebApi.Client .

Answer 6

Ok, it is apocalyptical 2020 now, and you can find these methods in NuGet package System.Net.Http.Json . But beware that it uses System.Text.Json internally.

And if you really need to find out which API resides where, just use https://apisof.net/

Answer 7

我知道这个回复为时已晚，我遇到了同样的问题，我正在添加System.Net.Http.Formatting.Extension Nuget，在这里和那里检查后我发现添加了 Nuget 但System.Net.Http.Formatting.dll没有添加到引用中，我只是重新安装了 Nuget

Answer 8

尝试在您的项目中安装 NuGet 包：Microsoft.AspNet.WebApi.Client：

Install-Package Microsoft.AspNet.WebApi.Client

Answer 9

If you are already using Newtonsoft.Json try this:

  // Alternative using WebApi.Client 5.2.7
  ////var response = await Client.PutAsJsonAsync(
  ////    "api/AgentCollection", user
  ////    requestListDto)

  var response = await Client.PostAsync("api/AgentCollection", new StringContent(
    JsonConvert.SerializeObject(user), Encoding.UTF8, "application/json"));

Performance are better than JavaScriptSerializer. Take a look here https://www.newtonsoft.com/json/help/html/Introduction.htm

Answer 10

Instead of writing this amount of code to make a simple call, you could use one of the wrappers available over the internet.

I've written one called WebApiClient, available at NuGet... check it out!

https://www.nuget.org/packages/WebApiRestService.WebApiClient/

Answer 11

如果您在 Blazor 中玩耍并遇到错误，则需要添加包Microsoft.AspNetCore.Blazor.HttpClient 。

Answer 12

Just expanding Jeroen's answer with the tips in comments:

var content = new StringContent(
    JsonConvert.SerializeObject(user), 
    Encoding.UTF8, 
    MediaTypeNames.Application.Json);

var response = await client.PostAsync("api/AgentCollection", content);

Answer 13

For me I found the solution after a lot of try which is replacing

HttpClient

with

System.Net.Http.HttpClient

Answer 14

If you are using Blazor components and getting this error that means your component is not able to find the http client. use this line on top on your blazor component.

@inject HttpClient Http

Make sure you have this line in your Program or startup file

builder.Services.AddScoped(sp => 
new HttpClient { BaseAddress = new Uri(builder.HostEnvironment.BaseAddress) });

Answer 15

Based on the answers here talking about using Newtonsoft.Json, I created a helper class for this:

public class JsonContent : StringContent
{
    public JsonContent(object model) 
        : base(JsonConvert.SerializeObject(model), Encoding.UTF8, "application/json")
    {
    }
}

Example usage:

using (var content = new JsonContent(user))
{
    var response = client.PostAsync("api/AgentCollection", content).Result;
}