吃晚饭，迟了

Public Class frm_data
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdopen.Click
        Dim ds As New Data.DataSet()
        Dim dbaname As String = txtdba.Text '指定数据库文件路径及名称
        Dim connStr As String = "provider=Microsoft.Jet.OLEDB.4.0; Data Source=" & CurDir() & dbaname
        Dim tabname As String = cmbtable.Text
        Dim conn As OleDb.OleDbConnection = New OleDb.OleDbConnection(connStr)
        Dim cmd As OleDb.OleDbCommand = New OleDb.OleDbCommand
        Dim sql As String = "select * from " & tabname
        conn.Open()
        cmd.Connection = conn
        Dim da As OleDb.OleDbDataAdapter = New OleDb.OleDbDataAdapter(sql, conn)

        ds.Clear()
        da.Fill(ds, "pylj")
        DataGridView1.DataSource = Nothing
        Dim k As Integer = DataGridView1.ColumnCount - 1

        DataGridView1.DataSource = ds.Tables(0)
        DataGridView1.Refresh()
        Dim n As Integer = ds.Tables(0).Columns.Count - 1
        Dim y As Integer = cmbfiled.Items.Count - 1
        For x As Integer = 0 To y

            cmbfiled.Items.Clear()
        Next
        cmbfiled.Refresh()

        For i As Integer = 0 To n
            cmbfiled.Items.Add(ds.Tables(0).Columns(i).ColumnName)

        Next
        cmbfiled.SelectedIndex = 0
        conn.Close()
    End Sub

    Private Sub DataGridView1_DoubleClick(ByVal sender As Object, ByVal e As System.EventArgs) Handles DataGridView1.DoubleClick
        On Error Resume Next
        MsgBox(DataGridView1.CurrentCell.Value.ToString)
    End Sub

    Private Sub frm_data_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        txtdba.Text = "\data\py.mdb"
    End Sub

    Private Sub butconn_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles butconn.Click
        cmbtable.Items.Clear()
        For i As Int16 = 0 To GetAllTableName.Count - 1
            cmbtable.Items.Add(GetAllTableName.Item(i).ToString)
        Next

        cmbtable.SelectedIndex = 0

    End Sub


    Function GetAllTableName() As System.Collections.ArrayList
        Dim t As New System.Collections.ArrayList()
        Dim Rs As New ADODB.Recordset()
        Dim dbaname As String = txtdba.Text  '指定数据库文件路径及名称
        Dim connStr As String = "provider=Microsoft.Jet.OLEDB.4.0; Data Source=" & CurDir() & dbaname
        Dim conn As New ADODB.Connection
        conn.Open(connStr)
        Rs = conn.OpenSchema(ADODB.SchemaEnum.adSchemaTables)
        Do While Not rs.EOF
            If rs("TABLE_TYPE").Value = "TABLE" Then
                t.Add(rs("TABLE_NAME").Value)
            End If
            rs.MoveNext()
            If rs.EOF Then Exit Do
        Loop
        Rs.Close()
        Rs = Nothing
        Return t
    End Function

    Private Sub cmbtable_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmbtable.SelectedIndexChanged
        Button1_Click(sender, e)
    End Sub

End Class

另外，创建的对象应当及时关闭或销毁以释放内存，我并没有加入这些代码，养成良好的习惯才好

谢谢热心帮助
问题出在Public ds As New Data.DataSet() 上了，是吗？
现在cmbfiled_SelectedIndexChanged,显示该字段的数据类型，现在需要重新打开数据表了，这种方法我实现了
Public Class frm_data
    'Private ds As New Data.DataSet()
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdopen.Click
        Dim ds As New Data.DataSet()
        Dim dbaname As String = txtdba.Text '指定数据库文件路径及名称
        Dim connStr As String = "provider=Microsoft.Jet.OLEDB.4.0; Data Source=" & CurDir() & dbaname
        Dim tabname As String = cmbtable.Text
        Dim conn As OleDb.OleDbConnection = New OleDb.OleDbConnection(connStr)
        Dim cmd As OleDb.OleDbCommand = New OleDb.OleDbCommand
        Dim sql As String = "select * from " & tabname
        conn.Open()
        cmd.Connection = conn
        Dim da As OleDb.OleDbDataAdapter = New OleDb.OleDbDataAdapter(sql, conn)

        ds.Clear()
        da.Fill(ds, "pylj")
        DataGridView1.DataSource = Nothing
        Dim k As Integer = DataGridView1.ColumnCount - 1
        'For j As Integer = 0 To k
        'DataGridView1.Columns.Clear()
        ' DataGridView1.Columns.Remove(DataGridView1.SelectedColumns(j))
        'Next
        DataGridView1.Refresh()
        MsgBox("d")
        DataGridView1.DataSource = ds.Tables(0)
        DataGridView1.Refresh()
        Dim n As Integer = ds.Tables(0).Columns.Count - 1
        Dim y As Integer = cmbfiled.Items.Count - 1
        For x As Integer = 0 To y

            cmbfiled.Items.Clear()
        Next
        cmbfiled.Refresh()

        For i As Integer = 0 To n
            cmbfiled.Items.Add(ds.Tables(0).Columns(i).ColumnName)

        Next

        cmbfiled.SelectedIndex = 0
    End Sub


    Private Sub DataGridView1_CellContentClick(ByVal sender As System.Object, ByVal e As System.Windows.Forms.DataGridViewCellEventArgs) Handles DataGridView1.CellContentClick

    End Sub

    Private Sub DataGridView1_DoubleClick(ByVal sender As Object, ByVal e As System.EventArgs) Handles DataGridView1.DoubleClick
        MsgBox(DataGridView1.CurrentCell.Value)
    End Sub

    Private Sub frm_data_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        txtdba.Text = "\data\py.mdb"
    End Sub

    Private Sub butconn_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles butconn.Click
        cmbtable.Items.Clear()
        For i As Int16 = 0 To GetAllTableName.Count - 1
            cmbtable.Items.Add(GetAllTableName.Item(i).ToString)
            'MsgBox(GetAllTableName.Item(i).ToString)
        Next

        cmbtable.SelectedIndex = 0







    End Sub
   

    Private Sub cmbfiled_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmbfiled.SelectedIndexChanged
        Dim i As Integer = cmbfiled.SelectedIndex
        Dim ds As New Data.DataSet()
        Dim dbaname As String = txtdba.Text '指定数据库文件路径及名称
        Dim connStr As String = "provider=Microsoft.Jet.OLEDB.4.0; Data Source=" & CurDir() & dbaname
        Dim tabname As String = cmbtable.Text
        Dim conn As OleDb.OleDbConnection = New OleDb.OleDbConnection(connStr)
        Dim cmd As OleDb.OleDbCommand = New OleDb.OleDbCommand
        Dim sql As String = "select * from " & tabname
        conn.Open()
        cmd.Connection = conn
        Dim da As OleDb.OleDbDataAdapter = New OleDb.OleDbDataAdapter(sql, conn)

        ds.Clear()
        da.Fill(ds, "pylj")
        txtfieldtype.Text = ds.Tables(0).Columns(i).DataType.Name
    End Sub

    Function GetAllTableName() As System.Collections.ArrayList
        Dim t As New System.Collections.ArrayList()
        Dim Rs As New ADODB.Recordset()
        Dim dbaname As String = txtdba.Text  '指定数据库文件路径及名称
        Dim connStr As String = "provider=Microsoft.Jet.OLEDB.4.0; Data Source=" & CurDir() & dbaname
        Dim conn As New ADODB.Connection
        conn.Open(connStr)
        Rs = conn.OpenSchema(ADODB.SchemaEnum.adSchemaTables)
        Do While Not rs.EOF
            If rs("TABLE_TYPE").Value = "TABLE" Then
                t.Add(rs("TABLE_NAME").Value)
            End If
            rs.MoveNext()
            If rs.EOF Then Exit Do
        Loop
        Rs.Close()
        Rs = Nothing
        Return t
    End Function

End Class
现在我想把ds 定义成全局的，这样一句
txtfieldtype.Text = ds.Tables(0).Columns(i).DataType.Name
就可以
--------
现在我发现问题是ds里面数据没有清除，这个应该怎么办？？

感谢“不说也罢”，问题成功解决，现把改正后代码同大家一起分享

Imports System.Data.SqlClient
Imports System.Data
Public Class frm_data
    Private ds As New Data.DataSet()
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdopen.Click

        Dim dbaname As String = txtdba.Text '指定数据库文件路径及名称
        Dim connStr As String = "provider=Microsoft.Jet.OLEDB.4.0; Data Source=" & CurDir() & dbaname
        Dim tabname As String = cmbtable.Text
        Dim conn As OleDb.OleDbConnection = New OleDb.OleDbConnection(connStr)
        Dim cmd As OleDb.OleDbCommand = New OleDb.OleDbCommand
        Dim sql As String = "select * from " & tabname
        conn.Open() '打开连接
        cmd.Connection = conn
        Dim da As OleDb.OleDbDataAdapter = New OleDb.OleDbDataAdapter(sql, conn)
        ds.Tables.Clear()
        da.Fill(ds, "pylj")
        DataGridView1.DataSource = ds.Tables(0) '更换datagridview1的数据源
        'DataGridView1.Refresh() '这句不要datagridview1也可更新
        '--------------
        '获取表所含字段并更新至cmbfiled
        Dim n As Integer = ds.Tables(0).Columns.Count - 1
        cmbfiled.Items.Clear()
        For i As Integer = 0 To n
            cmbfiled.Items.Add(ds.Tables(0).Columns(i).ColumnName)
        Next
        cmbfiled.SelectedIndex = 0
    End Sub
    Private Sub DataGridView1_DoubleClick(ByVal sender As Object, ByVal e As System.EventArgs) Handles DataGridView1.DoubleClick
        MsgBox(DataGridView1.CurrentCell.Value)
    End Sub
    Private Sub frm_data_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        txtdba.Text = "\data\py.mdb"
    End Sub
    Private Sub butconn_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles butconn.Click
        cmbtable.Items.Clear()
        '显示数据所含表
        For i As Int16 = 0 To GetAllTableName.Count - 1
            cmbtable.Items.Add(GetAllTableName.Item(i).ToString)
        Next
        cmbtable.SelectedIndex = 0
    End Sub
    Private Sub cmbfiled_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmbfiled.SelectedIndexChanged
        Dim i As Integer = cmbfiled.SelectedIndex
        '显示所选字段数据类型
        txtfieldtype.Text = ds.Tables(0).Columns(i).DataType.Name
    End Sub

    Function GetAllTableName() As System.Collections.ArrayList
        Dim t As New System.Collections.ArrayList()
        Dim Rs As New ADODB.Recordset()
        Dim dbaname As String = txtdba.Text  '指定数据库文件路径及名称
        Dim connStr As String = "provider=Microsoft.Jet.OLEDB.4.0; Data Source=" & CurDir() & dbaname
        Dim conn As New ADODB.Connection
        conn.Open(connStr)
        Rs = conn.OpenSchema(ADODB.SchemaEnum.adSchemaTables)
        Do While Not Rs.EOF
            If Rs("TABLE_TYPE").Value = "TABLE" Then
                t.Add(Rs("TABLE_NAME").Value)
            End If
            Rs.MoveNext()
            If Rs.EOF Then Exit Do
        Loop
        Rs.Close()
        Rs = Nothing
        Return t
    End Function

End Class