用DataTable更新数据库???

已经存在共享的SQLConnection(conn)和 SQLDataAdapter(adapter)


Dim dt as new DataTable
Dim Dt_tmp as new DataTable

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

Query_sql = "select * from TableA"
conn.open()
dt.Clear()
adapter.SelectCommand = New SqlClient.SqlCommand(Query_sql, conn)
adapter.Fill(dt)
DataGrid1.DataSource = dt

End Sub


Private Sub btnSave_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSave.Click
DataGrid1
If conn.State = ConnectionState.Open then
Else
conn.open()
End If

' dt_tmp = dt_c_user.GetChanges(DataRowState.Modified)
adapter.Update(dt) '这个怎么不管用啊，
conn.Close()
End Sub

点btnSave,数据库无更新.
原因出在哪了？？？