Dialog Box WM_PAINT problem

[Elbios]

Hello everyone,
I'm studying an Object-Oriented Programming course and I'm supposed to write a small app in C++ which draws shapes based on given coordinates. I'm trying to do it in WINAPI (I use Dev C++) and I have a problem with the WM_PAINT message, I don't think I know exactly how it works.

I have a main window which is empty and serves as a canvas for drawing shapes, and a dialog box with boxes and buttons where you input data and initiate drawing. I completed the application and it works but it unfortunately uses up 25% of my CPU all the time. I noticed that if I remove one thing from the code, it then doesn't hog the resources but the shapes aren't drawn.

This is my message loop:

while ((bRet=GetMessage (&messages, NULL, 0, 0))!=0)
    {
        if (bRet==-1) return 0;
        else if (!IsWindow(toolbox) || !IsDialogMessage(toolbox, &messages))
        {
            /* Translate virtual-key messages into character messages */
            TranslateMessage(&messages);
            /* Send message to WindowProcedure */
            DispatchMessage(&messages);
        }
    }

This is the WinProc function:

LRESULT CALLBACK WindowProcedure (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{

    
    switch (message)                  /* handle the messages */
    {
        case WM_PAINT:  // this case uses 25% CPU if it's there, but doesn't draw shapes if I delete it
             break;
        case WM_DESTROY:
            PostQuitMessage (0);       /* send a WM_QUIT to the message queue */
            break;
        default:                      /* for messages that we don't deal with */
            return DefWindowProc (hwnd, message, wParam, lParam);
    }

    return 0;
}

I noticed that if i declare PAINTSTRUCT ps and do:
case WM_PAINT:
BeginPaint(hwnd,&ps);
EndPaint(hwnd,&ps);
break;

then it doesn't hog resources but no shapes can be drawn (canvas window remains clear);


this is my dialog box callback function(simplified):

INT_PTR CALLBACK toolboxProc(HWND hwndDlg, UINT message, WPARAM wParam, LPARAM lParam) 
{
            HDC hdc;
            HWND wzium; //not important
    PAINTSTRUCT ps;
    char* color=NULL;
    char colortemp[10];    
    HBRUSH brush;
    HPEN hpen=CreatePen(PS_NULL,1,RGB(0,0,0));
    RECT rect;
            
     switch (message)                  /* handle the messages */
     {
          
          
            case WM_INITDIALOG:
                 SendDlgItemMessage(hwndDlg,IDC_COMBO1,CB_ADDSTRING,0,(LPARAM)"triangle");
                 SendDlgItemMessage(hwndDlg,IDC_COMBO1,CB_ADDSTRING,0,(LPARAM)"circle");
                 SendDlgItemMessage(hwndDlg,IDC_COMBO1,CB_ADDSTRING,0,(LPARAM)"rectangle");  // add strings to combobox
                 
                    return TRUE;
          
            case WM_COMMAND:
                   if(HIWORD(wParam)==BN_CLICKED)
                   {
                       switch(LOWORD(wParam))
                       {
                                                                                
                            case IDC_BUTTON1: // the "draw" button
                                
                                 // here is code (i cut it out) which checks for different cases of strings in editbox and sets the color of the brush accordingly                              
                                
                                 hdc=BeginPaint(GetParent(hwndDlg),&ps);
                                 SelectObject(hdc,brush);
                                 SelectObject(hdc,hpen);
                                 if (SendDlgItemMessage(hwndDlg,IDC_COMBO1,CB_GETCURSEL,0,0)==0) //if triangle radio button is chosen
                                 {
                                                                                                 //put coordinates in a new triangle object
                                        triangle tr(GetDlgItemInt(hwndDlg,IDC_EDIT4,NULL,FALSE),
                                                    GetDlgItemInt(hwndDlg,IDC_EDIT6,NULL,FALSE),
                                                    GetDlgItemInt(hwndDlg,IDC_EDIT2,NULL,FALSE),
                                                    GetDlgItemInt(hwndDlg,IDC_EDIT5,NULL,FALSE),
                                                    GetDlgItemInt(hwndDlg,IDC_EDIT3,NULL,FALSE),
                                                    GetDlgItemInt(hwndDlg,IDC_EDIT7,NULL,FALSE));
                                        tr.draw(hdc);  //draws triangle (polygon)
                                 }
                                                    
                                 else if (SendDlgItemMessage(hwndDlg,IDC_COMBO1,CB_GETCURSEL,0,0)==1) //if circle is chosen
                                 {
                                        circle ci(GetDlgItemInt(hwndDlg,IDC_EDIT4,NULL,FALSE),
                                                  GetDlgItemInt(hwndDlg,IDC_EDIT6,NULL,FALSE),
                                                  GetDlgItemInt(hwndDlg,IDC_EDIT10,NULL,FALSE));
                                        ci.draw(hdc);
                                 }
                                 else if (SendDlgItemMessage(hwndDlg,IDC_COMBO1,CB_GETCURSEL,0,0)==2) //if rectangle is chosen
                                 {
                                        rectangle re(GetDlgItemInt(hwndDlg,IDC_EDIT4,NULL,FALSE),
                                                     GetDlgItemInt(hwndDlg,IDC_EDIT6,NULL,FALSE),
                                                     GetDlgItemInt(hwndDlg,IDC_EDIT8,NULL,FALSE),
                                                     GetDlgItemInt(hwndDlg,IDC_EDIT9,NULL,FALSE));
                                        re.draw(hdc);
                                 }   
                                    
                                 DeleteObject(brush);
                                 DeleteObject(hpen);
                                 EndPaint(GetParent(hwndDlg),&ps);
                                 InvalidateRect ( GetParent(hwndDlg), NULL, TRUE ); //i use this so that newly drawn shapes overwrite previous ones
                                 return TRUE;
                             case IDC_BUTTON2: //exit button
                                 //stuff here
                                 return TRUE;
                                 break;
                              
                                 
                            case IDOK1: //clear screen button
                                //stuff here
                                 return TRUE;
                       }
                 }
                 break;
     }
     return FALSE;
}

can someone explain to me how the WM_PAINT message works in this app and what can I do to make it draw shapes at 0% CPU use? thanks!

P.S I attached the program.