Private void RefreshCom ()//a Bug: sort of way is according to the character, not Numbers, so there will be a COM10 ahead of COM4, but problem solving: just don't sort, see program! 
{
CmbSerialPortIndex. Items. The Clear (); 
CmbSerialPortIndex. Text=string. The Empty; 

String [] ports=Win32api_findSerialPorts. MulGetHardwareInfo (Win32api_findSerialPorts. HardwareEnum. Win32_PnPEntity, "Name");//use WPI interface 



If (ports!=null) 
{
String [] rebuildPorts=new string [ports. Length]; 
For (byte I=0; I & lt; Ports. Length; I++)//the following I also feel also can optimize 
{
For (byte j=0; J & lt; Ports [I]. Length; J++) 
{
If ((ports [I] [j]=='(') 
& & Ports [I] [m + 1]=='C' 
& & (ports [I] [m + 2]=='O') 
& & Ports, + 3 [j] [I]=='M' 
) 
{
If (ports [I] + 5 [j]==') ')//(COMn) form 
{
RebuildPorts [I]="COM" + ports [I] [m + 4) + "" + ports [I] the Substring (0, j - 1); 
} 
Else if (ports [I] + 6 [j]==') ')//(COMnn) form 
{
RebuildPorts [I]="COM" + ports [I] [j + 4] + ports [I] + 5 [j] + "" + ports [I] the Substring (0, j - 1); 
} 
Else if (ports + 7 [j] [I]==') ')//(COMnnn) form 
{
RebuildPorts [I]="COM + ports" + + + 4 [j] [I] ports + 5 [j] + [I] ports [I] + 6 [j] + "" + ports [I] the Substring (0, j - 1); 
} 
Else if (ports [I] + 8 [j]==') ')//(COMnnnn) form 
{
RebuildPorts [I]="COM" + ports [I] [j + 4] + ports + 5 [j] + [I] ports + 5 [j] + [I] ports + 7 [j] [I] + "" + ports [I] the Substring (0, j - 1); 
} 


} 
} 

} 
CmbSerialPortIndex. Items. AddRange (rebuildPorts); 
CmbSerialPortIndex. SelectedIndex=cmbSerialPortIndex. Items. Count & gt; 0? 0:1; 
} 
The else//almost won't appear this kind of mistake 
{
MessageBox. Show (" find COM port error!" ); 
CmbSerialPortIndex. Text=null; 

} 
} 
/* * * * * * * * * * * * * * * * the following functions: in order to achieve the serial port can be based on USB events to detect update serial * * * * * * * * * * * * * * * * * * */
Public const int WM_DEVICECHANGE=0 x0219;//WM_DeviceChange 
Public enum WM_DEVICECHANGE_WPPARAMS 
{
DBT_DEVICEARRIVAL=0 x8000,//DeviceArray 
DBT_DEVICEREMOVECOMPLETE=0 x8004,//DeviceMoveComplete 
}; 

Protected override void WndProc (ref Message m) 
{
Try 
{
If (m.M sg==WM_DEVICECHANGE) 
{
The switch ((WM_DEVICECHANGE_WPPARAMS) (m. Param. ToInt32 ())) 
{
Case WM_DEVICECHANGE_WPPARAMS. DBT_DEVICEARRIVAL: 
Tmr_StrartRefreshCom. Enabled=Enabled; 
break; 
Case WM_DEVICECHANGE_WPPARAMS. DBT_DEVICEREMOVECOMPLETE://unloading 
Tmr_StrartRefreshCom. Enabled=Enabled; 
break; 
Default: 
break; 
} 
} 
} 
The catch (Exception ex) 
{
MessageBox. Show (ex. Message); 
} 
Base. WndProc (ref m); 
} 







When I want to perform, we don't understand how can appear System. ArgumentNullException: 'the Value always be null. 
The Parameter name: item '

I am using Visual studio 2019, 

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