SignatureValidation.cs

public string SignURL(string message, string secret)
{
    var encoding = new System.Text.UTF8Encoding();
    var keyBytes = encoding.GetBytes(secret);
    var messageBytes = encoding.GetBytes(message);
    using (var hmacsha1 = new HMACSHA256(keyBytes))
    {
        var hashMessage = hmacsha1.ComputeHash(messageBytes);
        return Convert.ToBase64String(hashMessage);
    }
}

public bool SignatureValidation(string query, string secret, string signature)
{
    return SignURL(HttpUtility.UrlDecode(query), secret).ToString().Equals(signature);
}