HWND FindWindowA(
  LPCSTR lpClassName,              窗口类名.字符串. 可以用Spy++获取
  LPCSTR lpWindowName           窗口名称
);
返回窗口句柄.                          

HDC GetDC(
  HWND hWnd                    传入设备对象.也就是窗口句柄
);

HPEN CreatePen(
  int      iStyle,                创建笔的风格, 意思就是你的画笔是实心的 还是虚线 还是其他.
  int      cWidth,              笔的宽度.如果实心的.笔的宽度则自定义设置.如果其他.不能超过1
  COLORREF color            RGB的颜色.  你的画笔是什么颜色的.是一串16进制可以在线取色
);
返回图形对象

HRGN CreateRectRgn(
  int x1,
  int y1,                             坐标
  int x2,
  int y2
);

HGDIOBJ SelectObject(  HDC hdc,          // handle to DC
  HGDIOBJ hgdiobj   // handle to object);

BOOL MoveToEx(
  HDC     hdc,
  int     x,
  int     y,
  LPPOINT lppt
);

HGDIOBJ GetStockObject(
  int i                                  对象的类型.你想从DC中获取什么对象类型.
);

COLORREF SetDCBrushColor(
  HDC      hdc,
  COLORREF color                   RGB颜色
);

#include "stdafx.h"
#include int main(int argc, char *argv[])
{
    
    //1.获取窗口句柄
    HWND hwnd = ::FindWindow(NULL, TEXT("计算器"));

    //2.获取DC.
    HDC hdc = ::GetDC(hwnd);

    //3.创建图形对象.
    HPEN hPen = CreatePen(PS_SOLID, 10, RGB(0xFF, 0, 0)); //设置为红色画笔

    //4.关联图形对象跟DC
    HPEN oldPen = (HPEN)::SelectObject(hdc, hPen); //返回旧的画笔
    
    //5.绘制直线.
    MoveToEx(hdc, 0, 0, NULL);
    LineTo(hdc, 600, 100);

    //6.释放资源.
    ::SelectObject(hdc, oldPen); //首先还原旧画笔
    DeleteObject(hPen);          //删除图形对象.
    ReleaseDC(hwnd,hdc);              //释放Dc

    system("pause");
    return 0;
}

#include "stdafx.h"
#include int main(int argc, char *argv[])
{
    
    //1.获取窗口句柄
    HWND hwnd = ::FindWindow(NULL, TEXT("计算器"));

    //2.获取DC.
    HDC hdc = ::GetDC(hwnd);

    //3.创建图形对象.
    HPEN hPen = CreatePen(PS_SOLID, 10, RGB(0xFF, 0, 0)); //设置为红色画笔
    HBRUSH hBru = (HBRUSH)::GetStockObject(DC_BRUSH);
    //4.关联图形对象跟DC
    HPEN oldPen = (HPEN)::SelectObject(hdc, hPen); //返回旧的画笔
    SetDCBrushColor(hdc, RGB(0, 0xFF, 0)); //设置画刷颜色
    HBRUSH oldBrush = (HBRUSH)::SelectObject(hdc, hBru);
    //5.绘制句柄
    Rectangle(hdc, 0, 0, 100, 100);

    //6.释放资源.
    ::SelectObject(hdc, oldPen); //首先还原旧画笔
    DeleteObject(hPen);          //删除图形对象.
    ReleaseDC(hwnd,hdc);              //释放Dc

    system("pause");
    return 0;
}