public static bool ValidateQuery(string query)
{
    return !ValidateRegex("delete", query) && !ValidateRegex("exec", query) && !ValidateRegex("insert", query) && !ValidateRegex("alter", query) &&
           !ValidateRegex("create", query) && !ValidateRegex("drop", query) && !ValidateRegex("truncate", query);
}
public static bool ValidateRegex(string term, string query)
{
    // this regex finds all keywords {0} that are not leading or trailing by alphanumeric
    return new Regex(string.Format("([^0-9a-z]{0}[^0-9a-z])|(^{0}[^0-9a-z])", term), RegexOptions.IgnoreCase).IsMatch(query);
}

public static bool IsDbAffected(string query, string conn, List<SqlParameter> parameters = null)
{
    var response = false;
    using (var sqlConnection = new SqlConnection(conn))
    {
        sqlConnection.Open();
        using (var transaction = sqlConnection.BeginTransaction("Test Transaction"))
        using (var command = new SqlCommand(query, sqlConnection, transaction))
        {
            command.Connection = sqlConnection;
            command.CommandType = CommandType.Text;
            command.CommandText = query;
            if (parameters != null)
                command.Parameters.AddRange(parameters.ToArray());
            // ExecuteNonQuery() does not return data at all: only the number of rows affected by an insert, update, or delete.
            if (command.ExecuteNonQuery() > 0)
            {
                transaction.Rollback("Test Transaction");
                response = true;
            }
            transaction.Dispose();
            command.Dispose();
        }
    }
    return response;
}

if (stringSql.Substring(0, 6).ToUpper() == "SELECT")
{
    //execute statement
}