httpclient.cs

HttpClient client = new HttpClient();
client.BaseAddress = new Uri(@"https://www.google.com.tw");

var response = await client.GetAsync(@"/");
response.Content.Headers.ContentType.CharSet = "utf-8";
//string result = await response.Content.ReadAsStringAsync(System.Text.Encoding.UTF8);
//string result = await response.Content.ReadAsStringAsync();
byte[] byteArray = await response.Content.ReadAsByteArrayAsync();
string result = System.Text.Encoding.UTF8.GetString(byteArray, 0, byteArray.Length);
result = System.Text.Encoding.GetEncoding("utf-8").GetString(byteArray);      
Console.WriteLine(System.Web.HttpUtility.UrlDecode(result));