按照綠界的檢查碼機制，我們要用程式來生成檢查碼

第一步的欄位排列，綠界已經示範給我們看了，就把裡面的數值替代就好

string input = "HashKey="+ appSettings["HashKey"] + "&ChoosePayment=Credit&EncryptType=1&ItemName=" + ItemName +"&MerchantID="+ appSettings["MerchantID"] + "&MerchantTradeDate="+ formattedDateTime + "&MerchantTradeNo="+ MerchantTradeNo + "&PaymentType=aio&ReturnURL="+ appSettings["ReturnURL"]+ "&TotalAmount="+ TotalAmount+ "&TradeDesc=" + TradeDesc + "&HashIV=" + appSettings["HashIV"];

然後另外建立一個方法，這個方法是為了進行URL encode替換，綠界要求使用.NET URLEncode 結果，所以要按照他們的轉換表重新寫一個程式
綠界提供的URL encode轉換表：
https://developers.ecpay.com.tw/?p=2904
綠界要求的是.NET URLEncode 結果這一列的轉換符號

我們簡單寫一個轉換程式，主要用替代方法把存入的字串做替代

public static string CheckMacValue(string data)
{
    string replacedString = data.Replace("%", "%25");

    replacedString = replacedString.Replace("~", "%7e");
    replacedString = replacedString.Replace("+", "%2b");
    replacedString = replacedString.Replace(" ", "+");
    replacedString = replacedString.Replace("@", "%40");
    replacedString = replacedString.Replace("#", "%23");
    replacedString = replacedString.Replace("$", "%24");
    replacedString = replacedString.Replace("&", "%26");
    replacedString = replacedString.Replace("=", "%3d");
    replacedString = replacedString.Replace(";", "%3b");
    replacedString = replacedString.Replace("?", "%3f");

    replacedString = replacedString.Replace("/", "%2f");
    replacedString = replacedString.Replace("\\", "%5c");
    replacedString = replacedString.Replace(">", "%3e");
    replacedString = replacedString.Replace("<", "%3c");
    replacedString = replacedString.Replace("`", "%60");
    replacedString = replacedString.Replace("[", "%5b");
    replacedString = replacedString.Replace("]", "%5d");
    replacedString = replacedString.Replace("{", "%7b");
    replacedString = replacedString.Replace("}", "%7d");
    replacedString = replacedString.Replace(":", "%3a");

    replacedString = replacedString.Replace("'", "%27");
    replacedString = replacedString.Replace("\"", "%22");
    replacedString = replacedString.Replace(",", "%2c");
    replacedString = replacedString.Replace("|", "%7c");


    //轉小寫
    replacedString = replacedString.ToLower();

    replacedString = CalculateSHA256(replacedString);

    return replacedString;
}

● urlencode轉換表：
https://developers.ecpay.com.tw/?p=2904

接著要使用之前介紹過的SHA256加密來產生雜凑值
因為Asp.Net Core也沒有內建SHA256加密的方法供我們使用，所以還是自己寫一個方法：

public static string CalculateSHA256(string input)
{
    using (SHA256 sha256 = SHA256.Create())
    {
        byte[] hashBytes = sha256.ComputeHash(Encoding.UTF8.GetBytes(input));
        StringBuilder builder = new StringBuilder();

        for (int i = 0; i < hashBytes.Length; i++)
        {
            builder.Append(hashBytes[i].ToString("X2")); // 将每个字节转换为两位的十六进制字符串
        }

        return builder.ToString();
    }
}