HttpClient not supporting PostAsJsonAsync method C#

Question

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).