vb.net ConnectionString 属性尚未初始化

        Dim conn As SqlConnection
        Dim da As SqlDataAdapter
        Dim ds As DataSet
        conn = New SqlConnection
        da = New SqlDataAdapter()
        ds = New DataSet


        conn.ConnectionString = "Data Source=20101127-0930;Initial Catalog=数据库名;Integrated Security=True"
        conn.Open()


        da.SelectCommand = New SqlCommand
        da.SelectCommand.Connection = New SqlConnection

        da.Fill(ds, ds.DataSetName)
        DataGridView1.DataSource = "数据表"
运行时显示 ConnectionString 属性尚未初始化