await运算符只能用于异步方法中。请考虑用async修饰符标记此方法，并将其返回类型更改为Task

        private void button1_Click(object sender, EventArgs e)
        {
            string url = "http://localhost:35234/api/Products";
            //创建HttpClient（注意传入HttpClientHandler）
            var handler = new HttpClientHandler()
            {
                AutomaticDecompression =System.Net.DecompressionMethods.GZip
            };
            using (HttpClient http = new HttpClient(handler))
            {
                //await异步等待回应
                HttpResponseMessage response = await http.GetAsync(url);
                //确保HTTP成功状态值
                response.EnsureSuccessStatusCode();

                //await异步读取最后的JSON（注意此时gzip已经被自动解压缩了，因为上面的AutomaticDecompression = DecompressionMethods.GZip）
                Console.WriteLine(await response.Content.ReadAsStringAsync());
            }
        }

出现错误，按提示进行修改

        private async void button1_Click(object sender, EventArgs e)