Asp.net 将数据集指定为dropdownlist的数据源时出现NullReferenceException

我想分配gridview控件中dropdownlist的数据源。但是当我执行下面的代码时，我得到了NullReferenceException

Protected Sub grvStudent_DataBound(ByVal sender As Object, ByVal e As GridViewRowEventArgs)
    If e.Row.RowType = DataControlRowType.DataRow Then
        Try
            Dim Connection As SqlConnection = New SqlConnection(ConnectionString)
            Dim Query As String = "select Course from Courses"
            Dim Command As SqlCommand
            Command = New SqlCommand(Query, Connection)
            Dim Da As New SqlDataAdapter(Command)
            Dim Ds As New DataSet()
            Connection.Close()
            Dim ddlCourse = DirectCast(e.Row.FindControl("ddlCourse"), DropDownList)
            Da.Fill(Ds)
            ddlCourse.DataSource = Ds    //Exception is here
            ddlCourse.DataTextField = "Course"
            ddlCourse.DataValueField = "Id"
            ddlCourse.DataBind()
        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try
    End If
End Sub

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在填充数据集之前要关闭连接，而且还没有打开连接，因此请先打开连接，然后填充数据集，然后才能关闭连接

Protected Sub grvStudent_DataBound(ByVal sender As Object, ByVal e As GridViewRowEventArgs)
If e.Row.RowType = DataControlRowType.DataRow Then
    Try
        Dim Connection As SqlConnection = New SqlConnection(ConnectionString)
        Dim Query As String = "select Course from Courses"
        Dim Command As SqlCommand
        Command = New SqlCommand(Query, Connection)
        Dim Da As New SqlDataAdapter(Command)
        Dim Ds As New DataSet()
        Connection.Open()
        Dim ddlCourse = DirectCast(e.Row.FindControl("ddlCourse"), DropDownList)
        Da.Fill(Ds)
        ddlCourse.DataSource = Ds    //Exception is here
        ddlCourse.DataTextField = "Course"
        ddlCourse.DataValueField = "Id"
        ddlCourse.DataBind()

    Catch ex As Exception
        MsgBox(ex.ToString)
    Finally
        Connection.Close()
    End Try

End If
End Sub

编辑： 添加此行并运行

   If (ds.Tables.Count > 0) Then

    //your binding code....

  Else

     MsgBox(ex.ToString)

  End If

供参考

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确保您的下拉列表位于HTML代码中网格的ItemTemplate部分，您可以仅在编辑部分使用它

       <asp:TemplateField HeaderText="Course">
            <EditItemTemplate>
                     <asp:DropDownList ID="ddlCourse" runat="server">
                    </asp:DropDownList>
            </EditItemTemplate>
            <ItemTemplate>
                 <asp:DropDownList ID="ddlCourse" runat="server">
                    </asp:DropDownList>
                <asp:Label ID="lblCourse" runat="server"  />
            </ItemTemplate>
     </asp:TemplateField>

我试过这个。但在那一行也有同样的例外。我的课程字段中有5个项目，我将得到5个错误消息框。您是否检查了我在答案中添加的链接？有与您的问题相关的完整示例。请尝试ddlCourse.DataSource=Ds.Tables[0]；对于例外行，我尝试了链接到@Meherzad。从昨天开始，我就一直这样，我访问了最多的网站，但这是一个奇怪的问题。我也使用了Ds.Tables（0）。但结果是一样的。尝试调试代码检查数据集是否包含any表在这里您可以看到示例或查看我编辑的代码