对象不能从 DBNull 转换为 C# 中的其他类型异常

Question

我正在使用 MySql 数据库，如果我执行解决方案，我会在 catch 块中收到异常，下面是我的代码MySqlManager dac = new MySqlManager();long value = 0;DbCommand dbCommand = dac.GetStoredProcCommand("Spforgotpassword");dac.AddOutParameter(dbCommand, "p_op_EmailId", DbType.Int32, -1);dac.AddInParameter(dbCommand, "p_p_EmailId", DbType.String, data.Email_Id);dac.AddInParameter(dbCommand, "p_p_Password", DbType.String, Generatehash256(data.Password));dac.AddInParameter(dbCommand, "p_P_EncryptedPassword", DbType.String, Encryption.Encrypt(data.Password));dac.AddInParameter(dbCommand, "p_P_Mode", DbType.String, 102);dac.AddInParameter(dbCommand, "p_IPAddress", DbType.String, data.IPAddress);IDataReader reader = dac.ExecuteReader(dbCommand);if (reader.Read()){    value = reader["result"] == DBNull.Value ? default(long) : Convert.ToInt32(reader["result"]);}dac.CloseConnection(dbCommand, reader);if (Convert.ToBoolean(ConfigurationSettings.AppSettings["EnableEmail"])){    if (Convert.ToInt32(dac.GetParameterValue(dbCommand, "p_op_EmailId")) != -1)    {        Mail.WebMail oWebmail = new Mail.WebMail(dac.GetParameterValue(dbCommand, "p_op_EmailId").ToString());    }}return value;一旦我执行这一行，我就会收到异常 --- Object cannot be cast from DBNull to other typesdac.GetParameterValue(dbCommand, "p_op_EmailId")该p_op_EmailId是输出参数。

Answer 1

看起来存储过程将值设置为p_op_EmailIdto null。所以你不能Int32直接将它转换为。

相反，您可以将其转换为一个可为空的 int，并-1在比较之前将其合并：

(dac.GetParameterValue(dbCommand, "p_op_EmailId") as int? ?? -1) != -1