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我的代码：

    Imports System.Data.OleDb
Public Class DBase

    Dim conn As New OleDbConnection
    Dim myqry As String = Nothing
    Dim mycmd As New OleDbCommand
    Dim mydr As OleDbDataReader
    Dim str As String = Nothing
    Dim objCmd As New OleDb.OleDbCommand

    Private Sub DBase_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Call LVsettings()
        Call ConnToDB()
        Call FillListview()
        Call Set1()
    End Sub
#Region "FillListView"

    Sub FillListview()
        LV.Items.Clear()
        myqry = "SELECT * from Accounts ORDER BY YrandSec,FirstName Asc"
        mycmd = New OleDbCommand(myqry, conn)
        mydr = mycmd.ExecuteReader

        While mydr.Read
            With LV
                .Items.Add(mydr("Number"))
                With .Items(.Items.Count - 1).SubItems
                    .Add(mydr("StudNo"))
                    .Add(mydr("FirstName"))
                    .Add(mydr("LastName"))
                    .Add(mydr("YrandSec"))
                    .Add(mydr("Exer1")) - Error occurs here
                    .Add(mydr("Exer2"))
                    .Add(mydr("Exer3"))
                    .Add(mydr("Exer4"))
                    .Add(mydr("Exer5"))
                End With
            End With
        End While
    End Sub

#End Region

错误：

ERROR - (Overload resolution failed because no Public 'Add' can be called with these arguments:
    'Public Function Add(text As String) As System.Windows.Forms.ListViewItem.ListViewSubItem':
        Argument matching parameter 'text' cannot convert from 'DBNull' to 'String'.
    'Public Function Add(item As System.Windows.Forms.ListViewItem.ListViewSubItem) As System.Windows.Forms.ListViewItem.ListViewSubItem':
        Argument matching parameter 'item' cannot convert from 'DBNull' to 'ListViewSubItem'.)
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1 回答 1

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Add 方法需要一个字符串参数，但单元格值为 DBNull。VB.NET 无法在运行时将 DBNull 转换为字符串。您可以执行以下操作之一： 1. 更改您的 SQL 语句以明确列出要选择的列。可能为 NULL 的列应该被 ISNULL 语句包围，例如 ISNULL('Exer1', '')。2. 更改 While 循环中的代码以处理 NULL 值，例如：

.Add(mydr("Exer1"))

To（colIndex 是结果中列的索引，不幸的是您不能使用列的名称）：

If Not mydr.IsDBNull(colIndex) Then .Add(mydr("Exer1"))

或类似的处理 NULL 值的东西。

于 2013-10-29T08:34:18.463 回答