await運算符只能用於異步方法中。請考慮用async修飾符標記此方法，並將其返回類型更改為Task

        private void button1_Click(object sender, EventArgs e)
        {
            string url = "http://localhost:35234/api/Products";
            //創建HttpClient（注意傳入HttpClientHandler）
            var handler = new HttpClientHandler()
            {
                AutomaticDecompression =System.Net.DecompressionMethods.GZip
            };
            using (HttpClient http = new HttpClient(handler))
            {
                //await異步等待回應
                HttpResponseMessage response = await http.GetAsync(url);
                //確保HTTP成功狀態值
                response.EnsureSuccessStatusCode();

                //await異步讀取最后的JSON（注意此時gzip已經被自動解壓縮了，因為上面的AutomaticDecompression = DecompressionMethods.GZip）
                Console.WriteLine(await response.Content.ReadAsStringAsync());
            }
        }

出現錯誤，按提示進行修改

 

        private async void button1_Click(object sender, EventArgs e)