'Task <HttpResponseMessage>'不包含'GetAwaiter'的定义,也没有扩展方法'GetAwaiter'

Question

我HttpClient在一个类库项目中使用(.net4.0).为了实现这个目的,我为Bcl添加了nuget包

<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="Microsoft.AspNet.WebApi.Client" version="4.0.30506.0" targetFramework="net40" />
  <package id="Microsoft.Bcl" version="1.1.10" targetFramework="net40" />
  <package id="Microsoft.Bcl.Async" version="1.0.168" targetFramework="net40" />
  <package id="Microsoft.Bcl.Build" version="1.0.14" targetFramework="net40" />
  <package id="Microsoft.Net.Http" version="2.2.29" targetFramework="net40" />
  <package id="Newtonsoft.Json" version="4.5.6" targetFramework="net40" />
</packages>

但我仍然收到错误消息

"严重程序代码说明项目文件行抑制状态错误CS1061'任务'不包含'GetAwaiter'的定义,并且没有扩展方法'GetAwaiter'接受类型'任务'类型的第一个参数(您是否缺少使用指令或装配参考？)"

我正在使用的代码如下

private async Task<bool> Login()
{
    try
    {
        var status = false;
        using (HttpClient client = new HttpClient())
        {

            client.DefaultRequestHeaders.Add("username", _userName);
            client.DefaultRequestHeaders.Add("password", _password);

            using (HttpResponseMessage response = await client.PostAsync(_url, null))
            {
                if (response.StatusCode == System.Net.HttpStatusCode.OK)
                {
                    var contents = await response.Content.ReadAsStringAsync();
                    AuthenticationResult result = JsonHelper.DeserializeJson<AuthenticationResult>(contents);
                    if (result != null) _authToken = result.authToken;
                }
            }
        }
        return status;
    }
    catch (Exception ex)
    {
        return false;
    }

}

奇怪的是,如果我将项目输出类型更改为Windows Application错误消失,这将开始工作.此错误仅在项目为a时出现Class Library