LINQ查询中DateTime为null的转换问题

以下是我在linq查询中使用的数据表的配置：我有两个数据集文件（所有表的所有列都指定了DataType，并且它们的AllowDbNull属性设置为True）：*deposit_position_inbalance.xsd：包含2个数据表：-不平衡-实时不平衡细节*dep_pos_imbalance_detail.xsd：包含1个数据表：-表

在下面的代码中，问题出现在"deal_date=b.deal_date"这两行。事实上，当我从数据库b.deal_date中检索到一个空值时，它在deposit_position_inbalance.Designer.cs中说："用户代码未处理StrongTypeingException"表'ImbalanceDetailForRealTime'中列'deal_date'的值为DBNull。"指定的强制转换无效"。以下是它抛出错误的地方：

[global:：System.Diagnostics.Degger NonUserCodeAttribute（）]公共System.DateTime deal_date{得到{尝试{return（（global:：System.DateTime）（this[this.tableImbalanceDetailForRealTime.dell_dateColumn]）；}catch（global:：System.InvalidCastException e）{throw new global:：System.Data.StrungTypeingException（"表'''ImplanceDetailForRealTime'''中列'''deal_date'''的值为DBNull。"+"，e）//此处引发错误}}集合{this[this.tableImbalanceDetailForRealTime.deleteColumn]=值；}}

• 我已尝试将行"deal_date=b.deal_date"替换为"deal_date=（DateTime？）b.deal_date"但我得到了两个编译错误："dep_pos_imbalance_detail.TableDataTable.AddTableRow（string，System.DateTime）的最佳重载方法匹配有一些无效参数"answers"参数"2"：无法从"System.DateTime？"转换为"System.DateTime'"
• 我还尝试将"deal_date=b.deal_date"一行替换为"deal_date=b.deal_date==null？（DateTime）DBNull.值：b.deal_date"但我得到了一个编译错误："无法将类型"System.DBNull"转换为System.DateTime'"
• 然后，我尝试将行"deal_date=b.deal_date"替换为"deal_date=b.deal_date==null？（DateTime？）DBNull.值：b.deal_date"但我得到了一个编译错误："无法将类型"System.DBNull"转换为System.DateTime？"
• 我尝试了另一件事：将"deal_date=b.deal_date"替换为"deal_date=b.Isdeal_dateNull（）？默认值（DateTime？）：b.deal_date"但是，我又犯了以下错误：dep_pos_imbalance_detail.TableDataTable.AddTableRow（string，System.DateTime）'的最佳重载方法匹配包含一些无效参数和"参数"2"：无法从"System.DateTime？"转换为"System.DateTime'"下图（很抱歉，我还不被允许在stackoverflow中插入图像，所以我放了链接）显示了我的数据集中我的列deal_date的定义：https://lh5.googleusercontent.com/-TEZZ9Hdnkl4/T1aRxF_i7II/AAAAAAAAAAg/BwzrVXIlOHE/s323/deal_date.jpg我们可以看到，我似乎没有设置"System.DateTime？"的可能性，而只设置"System.DateTime"。我不希望将null以外的任何值作为默认值（我们是否必须放置默认值"以外的其他值才能使其工作？）UPDATE-->我尝试将null而不是，但设计器给出了以下错误："该字符串未被识别为有效的DateTime。有一个未知单词从索引0开始。"

所以我不明白如何检索null值（我没有把它放在代码中，但我对类型double也有同样的问题）。我的印象是，我的列被设置为启用null值，但显然不是。。。此外，当我试图将NullValue属性从"（Throw Exception）"修改为"（Empty）"或"（Null）"时，设计器会给出以下错误："输入的值对当前数据类型无效。"谢谢你的帮助。这是我的LINQ查询：

deposit_position_imbalance.ImbalanceDataTable dtImbalanceForRealTime;
deposit_position_imbalance.ImbalanceDetailForRealTimeDataTable dtImbalanceDetailForRealTime;
dtImbalanceForRealTime = (deposit_position_imbalance.ImbalanceDataTable)(((deposit_position_imbalance)(dataManager.GetConfig(grid1).ParentDataSource)).Imbalance);
dtImbalanceDetailForRealTime = this.detailForRealTime;
// we separate security_id null and not null
// Security id is not null
deposit_position_imbalance.ImbalanceDataTable iWithSecurityIdNotNull = new deposit_position_imbalance.ImbalanceDataTable();
deposit_position_imbalance.ImbalanceRow[] dr1 = (deposit_position_imbalance.ImbalanceRow[])dtImbalanceForRealTime.Select("security_id is not null");
if (dr1.Count<deposit_position_imbalance.ImbalanceRow>() > 0)
{
    DataTable looselyTypedDT1 = dr1.CopyToDataTable<deposit_position_imbalance.ImbalanceRow>();
    iWithSecurityIdNotNull.Merge(looselyTypedDT1, true);
}
// Security id is null
deposit_position_imbalance.ImbalanceDataTable iWithSecurityIdNull = new deposit_position_imbalance.ImbalanceDataTable();
deposit_position_imbalance.ImbalanceRow[] dr2 = (deposit_position_imbalance.ImbalanceRow[])dtImbalanceForRealTime.Select("security_id is null");
if (dr2.Count<deposit_position_imbalance.ImbalanceRow>() > 0)
{
    DataTable looselyTypedDT2 = dr2.CopyToDataTable<deposit_position_imbalance.ImbalanceRow>();
    iWithSecurityIdNull.Merge(looselyTypedDT2, true);
}
var queryWithSecurityIdFound =
    from a in iWithSecurityIdNotNull
    join b in dtImbalanceDetailForRealTime
    on new
    {
        a.situation_date,
        a.security_id,
        a.deposit_location_id,
        a.account_keeper_id
    }
        equals new
        {
            b.situation_date,
            b.security_id,
            b.deposit_location_id,
            b.account_keeper_id
        }
    where a.situation_date == situation_date
       && a.security_id == security_id
       && a.deposit_location_id == deposit_location_id
       && a.account_keeper_id == account_keeper_id
    select new
    {
        name = a.bo_source_name,
        deal_date = b.deal_date
    };
var queryWithSecurityIdNotFound =
    from a in iWithSecurityIdNull
    join b in dtImbalanceDetailForRealTime
        on new
        {
            a.situation_date,
            a.security_code,
            a.deposit_location_id,
            a.account_keeper_id
        }
        equals new
        {
            b.situation_date,
            b.security_code,
            b.deposit_location_id,
            b.account_keeper_id
        }
    where a.situation_date == situation_date
       && a.security_id == security_id
       && a.deposit_location_id == deposit_location_id
       && a.account_keeper_id == account_keeper_id
    select new
    {
        name = a.bo_source_name,
        deal_date = b.deal_date
    };
var query_final = queryWithSecurityIdFound.Union(queryWithSecurityIdNotFound);
//We fill the 'dep_pos_imbalance_detail Table'
grid1.Clear();
foreach (var item in query_final)
{
    ((dep_pos_imbalance_detail.TableDataTable)grid1.DataSet.Tables["Table"]).AddTableRow(item.name, item.deal_date);
}

if (!row.Isdeal_dateNull)
{
   //do something
}

dep_pos_imbalance_detail.TableDataTable dt = ((dep_pos_imbalance_detail.TableDataTable)grid1.DataSet.Tables["Table"]);
dep_pos_imbalance_detail.TableRow dr = dt.NewTableRow();
foreach (var item in query_final)
{
   dr = dt.NewTableRow();
   dr.name = item.name;
   if (item.deal_date.HasValue)
       dr.deal_date = item.deal_date.Value;
   else
       dr.Setdeal_dateNull();
   dt.AddTableRow(dr);
}