HttpClient not supporting PostAsJsonAsync method C# – Dev

The best answers to the question “HttpClient not supporting PostAsJsonAsync method C#” in the category Dev.


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/");
   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.


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.

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


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



Yes, you need to add a reference to


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.


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).


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.