The type or namespace name 'Http' does not exist in the namespace 'System.Web' - ASP.net asmx

Question

I'm trying to write a method to handle Facebook webhooks challenge as so:

using System.Web;
using System.Web.Http;    
[HttpGet]    
    public HttpResponseMessage Get()
            {
                var response = new HttpResponseMessage(HttpStatusCode.OK)
                {
                    Content = new StringContent(HttpContext.Current.Request.QueryString["hub.challenge"])
                };
                response.Content.Headers.ContentType = new MediaTypeHeaderValue("text/plain");
                return response;
            }

When building the project, I receive an error which states that "Http" does not exists in the namespace "System.Web" (and with it "HttpGet"). I tried reinstalling Microsoft.AspNet.WebApi through NuGet console but saw no change. Also tried adding this to the packages.config file:

<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="Microsoft.AspNet.WebApi" version="5.2.3" targetFramework="net452" />
  <package id="Microsoft.AspNet.WebApi.Client" version="5.2.3" targetFramework="net452" />
  <package id="Microsoft.AspNet.WebApi.Core" version="5.2.3" targetFramework="net452" />
  <package id="Microsoft.AspNet.WebApi.WebHost" version="5.2.3" targetFramework="net452" />
  <package id="Microsoft.CodeDom.Providers.DotNetCompilerPlatform" version="1.0.8" targetFramework="net45" />
  <package id="Microsoft.Net.Compilers" version="2.6.0" targetFramework="net45" developmentDependency="true" />
  <package id="MSBuildTasks" version="1.5.0.235" targetFramework="net452" developmentDependency="true" />
  <package id="Newtonsoft.Json" version="10.0.3" targetFramework="net45" />
</packages>

web.config file:

    <?xml version="1.0"?>
<configuration>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="Newtonsoft.Json" publicKeyToken="30ad4fe6b2a6aeed" culture="neutral"/>
        <bindingRedirect oldVersion="0.0.0.0-10.0.0.0" newVersion="10.0.0.0"/>
      </dependentAssembly>
    </assemblyBinding>
    <assemblyBinding>
      <dependentAssembly>
        <assemblyIdentity name="System.Web.Http" publicKeyToken="31bf3856ad364e35" culture="neutral"/>
        <bindingRedirect oldVersion="0.0.0.0-4.5.0.0" newVersion="5.0.0.0"/>
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
  <system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.8.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701"/>
      <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.8.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:14 /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+"/>
    </compilers>
  </system.codedom>
  <!--
    For a description of web.config changes see http://go.microsoft.com/fwlink/?LinkId=235367.

    The following attributes can be set on the <httpRuntime> tag.
      <system.Web>
        <httpRuntime targetFramework="4.5.2" />
      </system.Web>
  -->
  <system.web>
    <compilation targetFramework="4.5"/>
  </system.web>
</configuration>

Thanks for your help :)