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请帮助....当我从Mysql表中选择数据时,它显示"已经有一个与此Connection相关的开放DataReader必须先关闭.vb.net"

Private Sub cmbJobCategoryVisa_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmbJobCategoryVisa.SelectedIndexChanged
    ''"
    Dim MyCommand As New MySqlCommand("SELECT jobcategorycode FROM jobcategory WHERE jobcategory='" & Me.cmbJobCategoryVisa.SelectedItem & "'", MyConnection)
    Dim MyReader As MySqlDataReader = MyCommand.ExecuteReader
    While MyReader.Read
        If MyReader.HasRows = True Then
            Me.txtJobCategoryCodeVisa.Text = MyReader("jobcategorycode")
        End If
    End While
    MyReader.Close()
    MyCommand.Dispose()
End Sub

'''在下面的代码执行时,',显示成像错误

    Private Sub txtEmpNo_Validating(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles txtEmpNo.Validating
    Dim MyCommand5 As New MySqlCommand("SELECT * FROM employeesmaster WHERE empno='" & Me.txtEmpNo.Text & "'", …