HttpClient injected with Unity error in Web Forms page - Type or namespace name 'HttpClient' does not exist in the namespace 'System.Net.Http'

Question

I'm maintaining an old ASP.NET 4.8 Web Forms application and trying to inject HttpClient into a Page using Unity Container. My Global.asax.cs page looks thusly:

var container = this.AddUnity();
container.RegisterType<HttpClient>(new InjectionFactory(x => new HttpClient()));

(I've also tried registering it as a Singleton and a few other ways to register it, but that doesn't seem to be the issue.)

In the code behind on my page, I have the following code:

public partial class page : Page 
{
    [Dependency]
    private HttpClient _httpClient { get; }

    public page() { }

    public page(HttpClient client)
    {
        _httpClient = client;
    }
}

I have dependency injection working fine elsewhere, so I don't think it's Unity's fault. I also have HttpClient elsewhere on this page, so I'm using System.Net.Http; it's definitely installed. In fact, when I use HttpClient as a static object (not injected into the page), it works fine--so I know for sure System.Net.Http is installed as a reference and working fine.

Problem is, when I run the page, I get the following error:

The type or namespace name 'HttpClient' does not exist in the namespace 'System.Net.Http' (are you missing an assembly reference?) [System.Diagnostics.DebuggerNonUserCodeAttribute()] public shared_page_aspx(System.Net.Http.HttpClient client) : base(client) { this.@__Init();

Why? 😭 Where is this weird code coming from?

Answer 1

Changed to singleton:

container.RegisterSingleton<HttpClient>(new InjectionFactory(x => new HttpClient()));

And also added this to web.config:

<system.web>
    <compilation debug="true" targetFramework="4.8">
        <assemblies>
            <add assembly="netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51"/>
        </assemblies>
    </compilation>
</system.web>