无法使用来自Asp MVC 6 Web Api的HttpClient

我尝试编写简单的代码：

 public async Task<string> GetData(String labelName)
    {           
        using (var client = new HttpClient())
        {
            var uri = new Uri(@"https://example.com/over/search_field?=search_label=" + labelName);
            var response = await client.GetAsync(uri).ConfigureAwait(false);
            var textResult = await response.Content.ReadAsStringAsync();
            return textResult;
        }
    }

在project.json:

 {
"webroot": "wwwroot",
"version": "1.0.0-*",
"dependencies": {
  "Microsoft.AspNet.IISPlatformHandler": "1.0.0-beta8",
 "Microsoft.AspNet.Mvc": "6.0.0-beta8",
 "Microsoft.AspNet.Server.Kestrel": "1.0.0-beta8",
 "Microsoft.AspNet.StaticFiles": "1.0.0-beta8",
 "Microsoft.Framework.Logging": "1.0.0-beta8",
 "Microsoft.Framework.Logging.Console": "1.0.0-beta8",
 "Microsoft.Framework.Logging.Debug": "1.0.0-beta8",
 "Microsoft.Net.Http": "2.2.22"
  },      
 "frameworks": {
  "dnx451": {
    "dependencies": {
       "Microsoft.AspNet.WebApi.Client": "5.2.3",
      "Microsoft.AspNet.WebApi.Owin": "5.2.3",
      "System.Net.Http": "4.0.1-beta-23409"

   },
   "frameworkAssemblies": {
     "System.Net": "4.0.0.0",
     "System.Net.Http": "4.0.0.0"
   }
 },
 "dnxcore50": { }
},

但是，当我试图构建代码时，我得到了错误：

错误CS0246找不到类型或命名空间名称"HttpClient"（是否缺少using指令或程序集引用？）

你能帮我吗？如何修复此错误？我使用VS 2015和ASP MVC 6 Web API。非常感谢。

"dependencies": {
    "Microsoft.AspNet.Diagnostics": "1.0.0-rc1-final",
    "Microsoft.AspNet.IISPlatformHandler": "1.0.0-rc1-final",
    "Microsoft.AspNet.Mvc": "6.0.0-rc1-final",
    "Microsoft.AspNet.Mvc.TagHelpers": "6.0.0-rc1-final",
    "Microsoft.AspNet.Server.Kestrel": "1.0.0-rc1-final",
    // Removed some dependencies for saving space:)
    "Microsoft.VisualStudio.Web.BrowserLink.Loader": "14.0.0-rc1-final",
    "System.Net.Http": "4.0.1-beta-23409"
},
"frameworks": {
    "dnx451": { },
    "dnxcore50": {}
},

using System.Net.Http;