Delphi的Canvas为什么不允许进行绘图操作呢？

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我想要从整个屏幕绘制一个截图到TForm1的Canvas。此代码在Delphi XE3中运行良好。

procedure TForm1.Button1Click(Sender: TObject);var c, scr: TCanvas; r, r2: TRect;begin c :=TCanvas.Create; scr :=TCanvas.Create; c.Handle :=GetWend

我想从整个屏幕绘制一个截图到TForm1 Canvas.

此代码在Delphi XE3中运行良好

procedure TForm1.Button1Click(Sender: TObject); var c,scr: TCanvas; r,r2: TRect; begin c := TCanvas.Create; scr := TCanvas.Create; c.Handle := GetWindowDC(GetDesktopWindow); try r := Rect(0, 0, 200, 200); form1.Canvas.CopyRect(r, c, r); finally ReleaseDC(0, c.Handle); c.Free; end;

现在我想首先将屏幕截图复制到另一个画布.
有没有办法在没有出现此错误的情况下执行此操作？

procedure TForm1.Button1Click(Sender: TObject); var c,scr: TCanvas; r,r2: TRect; begin c := TCanvas.Create; scr := TCanvas.Create; c.Handle := GetWindowDC(GetDesktopWindow); try r := Rect(0, 0, 200, 200); scr.CopyRect(r,c,r); <-- Error, canvas does not allow drawing form1.Canvas.CopyRect(r, scr, r); <-- Error, canvas does not allow drawing finally ReleaseDC(0, c.Handle); c.Free; end; 如果您需要使用其他画布,则必须分配HDC,例如

var WindowHandle:HWND; ScreenCanvas,BufferCanvas: TCanvas; r,r2: TRect; ScreenDC,BufferDC :HDC; BufferBitmap : HBITMAP; begin WindowHandle := 0; ScreenCanvas := TCanvas.Create; BufferCanvas := TCanvas.Create; ScreenDC:=GetWindowDC(WindowHandle); ScreenCanvas.Handle := ScreenDC; BufferDC := CreateCompatibleDC(ScreenDC); BufferCanvas.Handle := BufferDC; BufferBitmap := CreateCompatibleBitmap(ScreenDC, GetDeviceCaps(ScreenDC, HORZRES), GetDeviceCaps(ScreenDC, VERTRES)); SelectObject(BufferDC, BufferBitmap); try r := Rect(0, 0, 200, 200); BufferCanvas.CopyRect(r,ScreenCanvas,r); form1.Canvas.CopyRect(r, BufferCanvas, r); finally ReleaseDC(WindowHandle, ScreenCanvas.Handle); DeleteDC(BufferDC); DeleteObject(BufferBitmap); BufferCanvas.Free; ScreenCanvas.Free; end; end;