c＃中的“无法将对象从DBNull强制转换为其他类型”

Question

我以前没有遇到过这种情况，但是我经历过的这种情况已经经历了多次。

例如，我有这种方法：

mySqlCommand = mySqlConnect.CreateCommand();
mySqlCommand.CommandText = "SELECT EquipmentNumber, EquipmentType, P.AreaCode AS Location, EquipmentDsc FROM MajorEquipment ME"
  + " INNER JOIN PlantAreaCodes P ON ME.PACId = P.PACId WHERE"
  + "(@EquipNumber IS NULL OR EquipmentNumber LIKE @EquipNumber)"
  + " AND ((SELECT PACId FROM PlantAreaCodes WHERE AreaCode = @Location) IS NULL OR ME.PACId = (SELECT PACId FROM PlantAreaCodes WHERE AreaCode = @Location))"
  + " AND (@EquipType IS NULL OR EquipmentType LIKE @EquipType)"
  + " AND (@Comments IS NULL OR EquipmentDsc LIKE @Comments);";

mySqlCommand.Parameters.Add("@EquipNumber", MySqlDbType.VarChar);
mySqlCommand.Parameters.Add("@Location", MySqlDbType.VarChar);
mySqlCommand.Parameters.Add("@EquipType", MySqlDbType.VarChar);
mySqlCommand.Parameters.Add("@Comments", MySqlDbType.VarChar);

mySqlCommand.Parameters["@EquipNumber"].Value = MeModel.EquipmentNumber;
mySqlCommand.Parameters["@Location"].Value = MeModel.Location;
mySqlCommand.Parameters["@EquipType"].Value = MeModel.EquipmentType;
mySqlCommand.Parameters["@Comments"].Value = MeModel.Notes;

mySqlReader = mySqlCommand.ExecuteReader();

MajorEquipment_Controller.MeList = new List<MajorEquipment_Model>();

while (mySqlReader.Read())
{
    Console.WriteLine(mySqlReader["EquipmentDsc"].GetType());
    MajorEquipment_Controller.MeList.Add(new MajorEquipment_Model
    {
        EquipmentNumber = Convert.ToString(mySqlReader["EquipmentNumber"]),
        EquipmentType = Convert.ToString(mySqlReader["EquipmentType"]),
        Location = Convert.ToString(mySqlReader["Location"]),
        Notes = Convert.ToString(mySqlReader["EquipmentDsc"])
    });
}
mySqlReader.Close();
mySqlCommand.ExecuteNonQuery();

在这种方法中， EquipmentDsc允许的是System.DBNull ，但我之前从未遇到过问题。

但是，在一个单独的程序中，我有一个类似的方法：

mySqlCommand = mySqlConnect.CreateCommand();
mySqlCommand.CommandText = "SELECT * FROM IT_Requests;";

mySqlReader = mySqlCommand.ExecuteReader();

IT_Admin.Admin_Controller.List = new List<IT_Admin.Admin_Model>();

while (mySqlReader.Read())
{
    MessageBox.Show((mySqlReader["Name"].GetType()).ToString());
    IT_Admin.Admin_Controller.List.Add(new IT_Admin.Admin_Model
    {
        ReqID = Convert.ToInt32(mySqlReader["ReqID"]),
        Issue = Convert.ToString(mySqlReader["Issue"]),
        PC = Convert.ToString(mySqlReader["PC"]),
        Date = Convert.ToString(mySqlReader["ReqDate"]),
        Name = Convert.ToString(mySqlReader["Name"]),
        ReqType = Convert.ToString(mySqlReader["ReqType"]),
        Urgency = Convert.ToInt32(mySqlReader["Urgency"]),
        Description = Convert.ToString(mySqlReader["Description"]),
        PrevReport = Convert.ToBoolean(mySqlReader["PrevReport"]),
        Completed = Convert.ToBoolean(mySqlReader["Completed"]),
        Resolution = Convert.ToString(mySqlReader["Resolution"]),
        ResDate = Convert.ToString(mySqlReader["ResDate"])
    });
}

并且一旦它达到一个空值，它就会吐出“ InvalidCastException”错误。

有谁知道是什么原因造成的？

Answer 1

Convert.ToString允许使用NULL DB值，因为System.String本身是可为空的引用类型，而System.Boolean为不可为空的值类型。

对您要读取的每个可为空的列执行此操作：

Object value = mySqlReader["columnName"];
if( value != DBNull.Value ) {
    destination = Convert.ToSomeStrongType( value );
} else {
    destination = null;
}

Answer 2

您所要做的就是提前检查DBNull。

例如

if (!mySqlReader["ReqID"] is DBNull)
{ 
    ReqID = Convert.ToInt32(mySqlReader["ReqID"]);
}

有关未来问题的建议：尝试将代码示例缩小到相关点。 在这种情况下，您提供了一些非常大的样本。