HttpClient not supporting PostAsJsonAsync method C#

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.

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.

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 PostAsJsonAsyncwith 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

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.

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).

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.

I know this reply is too late, I had the same issue and i was adding the System.Net.Http.Formatting.Extension Nuget, after checking here and there I found that the Nuget is added but the System.Net.Http.Formatting.dll was not added to the references, I just reinstalled the Nuget

Try to install in your project the NuGet Package: Microsoft.AspNet.WebApi.Client:

Install-Package Microsoft.AspNet.WebApi.Client

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/

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

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/

If you're playing around in Blazor and get the error, you need to add the package Microsoft.AspNetCore.Blazor.HttpClient.

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);

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

HttpClient

with

System.Net.Http.HttpClient