转换错误DBNull无法转换为long [英] Conversion error DBNull cannot be converted to long

问题描述

大家好，感谢您阅读我的帖子。我有以下代码，每隔一段时间我就会收到一个转换错误：DBNull无法转换为long。我使用以下代码：

 受保护  Sub  Calendar332_SelectionChanged（发件人作为 对象，e  As  EventArgs）
 ' 声明将查询数据库的SQL语句 
  Dim  sqlstring  As   String  =   SELECT firstname，lastname，startdate，starttime，endtime，COALESCE（hours，0）AS hours FROM小时工作WHERE startdate> ='& Calendar332.SelectedDate&  'AND startdate< ='& Calendar333.SelectedDate&  'ORDER BY startdate 
 
 
  Dim  strSQLconnection 作为 字符串 =  数据源= SQL Server; SERVER = 123456;初始目录= ABC; UID = abc123; PWD = 123; 
 
  Dim  sqlConnection 作为 新 Data.SqlClient.SqlConnection（strSQLconnection）
  Dim  sqlCommand  As   New  Data.SqlClient.SqlCommand（sqlstring，sqlConnection）
 
 sqlConnection.Open（）
  Dim  reader  As  Data.SqlClient.SqlDataReader = sqlCommand.ExecuteReader（）
 
 Gr idView7.Visible =  True  
 GridView7.DataSource = reader 
 GridView7.DataBind（）
 reader.Close（）
  Dim  totalhours 
  Dim  sqlstring1  As  字符串 =   SELECT firstname，lastname，SUM（hours ）AS totalhours FROM hoursworked WHERE startdate> ='& Calendar332.SelectedDate&  'AND startdate< ='& Calendar333.SelectedDate&  'group by firstname，lastname 
  Dim  sqlConnection1  As  新 Data.SqlClient.SqlConnection（strSQLconnection）
  Dim  sqlCommand1 作为 新 Data.SqlClient.SqlCommand（ sqlstring1，sqlConnection1）
 '  Dim sqlCommand1 As New Data.SqlClient.SqlCommand（sqlstring1，sqlConnection） 
 sqlConnection1.Open（）
  Dim  reader1  As  Data.SqlClient.SqlDataReader = sqlCommand1.ExecuteReader（）
 
 如果 totalhours   DBNull.Value 然后 
 totalhours =  0  
 结束 如果 
 
 
 GridView8.Visible =  True  
 GridView8.DataSource = reader1 
 GridView8.DataBind（） 
 reader1.Close（）
 结束  Sub  

在aspx部分我有：

 <   ItemTemplate  >  
 <％ #Format（（CLng（DataBinder.Eval（Container.DataItem，  totalhours）） - （CLng（DataBinder.Eval（Container.DataItem，  totalhours ））Mod  60 ））/  60 ，  0）
 ％>  
：
 <％ #Format（CLng（DataBinder.Eval（Container.DataItem，  totalhours））Mod  60 ，  00）％>  
 <   / ItemTemplate  < span class =code-keyword>>  

我在第一个select语句中使用了COALESCE后错误消失了远。但是，当我作为最终用户并开始随机点击内容时，错误返回。我希望我的if语句可以解决问题，但没有这样的运气。任何想法或建议将不胜感激。



非常感谢，

D-Bar

解决方案

Is运算符确定两个对象引用是否引用同一个对象。但是，它不执行值比较



我认为你想要一个等号（ = ），而不是是，但我不是VB专家。

Hello all, thanks for reading my post. I have the following code and every so often I get a conversion error: DBNull cannot be converted to long. I am using the following code:

Protected Sub Calendar332_SelectionChanged(sender As Object, e As EventArgs)
        'declare SQL statement that will query the database
        Dim sqlstring As String = "SELECT firstname, lastname, startdate, starttime, endtime, COALESCE(hours,0) AS hours FROM hoursworked WHERE startdate >= '" & Calendar332.SelectedDate & "' AND startdate <= '" & Calendar333.SelectedDate & "' ORDER BY startdate"
        

        Dim strSQLconnection As String = "Data Source=SQL Server;SERVER=123456; Initial Catalog=ABC; UID=abc123;PWD=123;"

        Dim sqlConnection As New Data.SqlClient.SqlConnection(strSQLconnection)
        Dim sqlCommand As New Data.SqlClient.SqlCommand(sqlstring, sqlConnection)

        sqlConnection.Open()
        Dim reader As Data.SqlClient.SqlDataReader = sqlCommand.ExecuteReader()

        GridView7.Visible = True
        GridView7.DataSource = reader
        GridView7.DataBind()
        reader.Close()
        Dim totalhours
        Dim sqlstring1 As String = "SELECT firstname, lastname, SUM(hours) AS totalhours FROM hoursworked WHERE startdate >= '" & Calendar332.SelectedDate & "' AND startdate <= '" & Calendar333.SelectedDate & "'  group by firstname, lastname"
        Dim sqlConnection1 As New Data.SqlClient.SqlConnection(strSQLconnection)
        Dim sqlCommand1 As New Data.SqlClient.SqlCommand(sqlstring1, sqlConnection1)
        '  Dim sqlCommand1 As New Data.SqlClient.SqlCommand(sqlstring1, sqlConnection)
        sqlConnection1.Open()
        Dim reader1 As Data.SqlClient.SqlDataReader = sqlCommand1.ExecuteReader()

        If totalhours Is DBNull.Value Then
            totalhours = 0
        End If


        GridView8.Visible = True
        GridView8.DataSource = reader1
        GridView8.DataBind()
        reader1.Close()
    End Sub

In the aspx part I have:

<ItemTemplate>
                        <%# Format((CLng(DataBinder.Eval(Container.DataItem, "totalhours")) - (CLng(DataBinder.Eval(Container.DataItem, "totalhours")) Mod 60)) / 60, "0")
                        %>
                        :
                        <%# Format(CLng(DataBinder.Eval(Container.DataItem, "totalhours")) Mod 60, "00")%>
                        </ItemTemplate>

After I used COALESCE in the first select statement the error went away. But, when I act as an end user and start clicking on stuff randomly the error returned. I was hoping my if statement would do the trick, but no such luck. Any ideas or suggestions would be greatly appreciated.

Thank you very much,
D-Bar

解决方案

"The Is operator determines if two object references refer to the same object. However, it does not perform value comparisons"

I think you want an equal sign (=), rather than is, but I'm no VB expert.

这篇关于转换错误DBNull无法转换为long的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！