组合框关系表没有显示数据

Question

我尝试添加关系，以显示有关combobox和datagridview的数据。我已经尝试过以下代码

Private Sub LoadSemester()
    Me.OpenConn()

    Dim dSet As New DataSet

    Dim sql1 As String = "SELECT * FROM tbl_semester"
    Dim comm1 As New SqlClient.SqlCommand(sql1, cnn)
    Dim daSemester As SqlClient.SqlDataAdapter

    Dim sql2 As String = "SELECT * FROM tbl_mk"
    Dim comm2 As New SqlClient.SqlCommand(sql2, cnn)
    Dim daMK As SqlClient.SqlDataAdapter

    daMK = New SqlClient.SqlDataAdapter(comm2)
    dSet.Clear()
    daMK.Fill(dSet, "tbl_mk")

    daSemester = New SqlClient.SqlDataAdapter(comm1)
    dSet.Clear()
    daSemester.Fill(dSet, "tbl_semester")
    dSet.Relations.Add("relation", dSet.Tables("tbl_semester").Columns("id_pk"), dSet.Tables("tbl_pk").Columns("id_pk"))

    With cmbSemester
        .DataSource = dSet.Tables("tbl_semester")
        .DisplayMember = "semester"
        .ValueMember = "id_semester"
        .SelectedIndex = 0
    End With

    'my datagridview here
End Sub

但它没有显示任何内容。但是当我删除'dSet.Relations.Add（“relation”，dSet.Tables（“tbl_semester”）。列（“id_pk”），dSet.Tables（“tbl_pk”）。列（“id_pk”））'组合框显示了数据。

我想出问题可能是dSet.Relation代码。仅供参考，我使用的是SQLServer 2005和VS Express 2012。