经过多少痛苦，我有一个能工作的函数。

After much anguish, I have a function that works.

public static string HmacSha256(string secretKey, string value)
{
    // Move strings to buffers.
    var key = CryptographicBuffer.ConvertStringToBinary(secretKey, BinaryStringEncoding.Utf8);
    var msg = CryptographicBuffer.ConvertStringToBinary(value, BinaryStringEncoding.Utf8);

    // Create HMAC.
    var objMacProv = MacAlgorithmProvider.OpenAlgorithm(MacAlgorithmNames.HmacSha256);
    var hash = objMacProv.CreateHash(key);
    hash.Append(msg);
    return CryptographicBuffer.EncodeToHexString(hash.GetValueAndReset());
}