serialport comport access and Acess to com port is denied.any solution

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C#

WPF

C#4.0

I have created separate class for Serialport and accessing open, close, portnames and Datarecieved event.Now my problem is dont want to hard code the Comport in Serialport class as below.1)Instead want to select one of the comport from combobox which is a getportnames().I have two tabItem,in first tabitem every time i open the port and close for every button click where as in second tabitem on button click i'll be reading data from serial port continuously by using DataReceived event in serialport.Oncei read data continously then i change to tabitem and i try to close the opened port and open it getting Access to comport i sdenied.why is it so and how i can solve it?? any idea ,any solution please let me know

//serialportclass

serialport sp =new Seriaport("COM1");(and other settings  too)

private void PortOpen()
{
sp.Open(); //access is denied
}

private void PortClose()
{
sp.Close();
}

public List<string> GetAllportNames()
{
List<string> lst =new List<string>();
foreach(var item in Serilport.GetPortNames())
{
lst.Add(item);
}
return lst;
}

In UI

In WindowLoaded Event

serialportclass spobj=new serialportclass();
Combobox1.ItemsSource=spobj.GetAllportNames(); //how can i access combobox selecteditem to serialportclass's COMport as show in above....