// 传入参数 url 为请求地址
async Task Get(string uri = "http://api.domain.com/search";)
{
    // 使用 using 语法建立一个明确周期的 HttpClient 实例
    using (HttpClient client = new HttpClient())
    {
        try
        {
            // 设置超时限制，有效防止浪费资源
            client.Timeout = TimeSpan.FromSeconds(30);
            //使用 GetAsync 进行异步 HTTPGET 请求
            HttpResponseMessage response = await client.GetAsync(uri);
            // 判断服务器响应代码是否为 2XX
            response.EnsureSuccessStatusCode();
            //使用 await 语法读取响应内容
            string responseBody = await response.Content.ReadAsStringAsync();
 
            Console.WriteLine(responseBody);
        }
        catch (HttpRequestException e)
        {
            Console.WriteLine("Error Message :{0} ", e.Message);
        }
    }
}    

async Task Get(string uri = "http://api.domain.com/search";)
{
    using (HttpClient client = new HttpClient())
    {
        HttpRequestMessage httpRequest = new HttpRequestMessage(HttpMethod.Get, uri);
 
        await client.SendAsync(httpRequest)
                .ContinueWith(responseTask =>
                    {
                        Console.WriteLine("Response: {0}", responseTask.Result);
                    });
    }   
}