关于 WPF 透明窗口的内存占用

要实现一个透明的 WPF 窗口?
What an easy task! By setting AllowTransparency and WindowStyle, you could finish it in seconds.

AllowTransparency="True" WindowStyle="None" Background="Transparent"

Correct? Of course.

However (you know this is coming), look at your task manager, easy task comes with large memory consumption, especially for 4K transparent window. 100+ MB ram are wasted for just showing an empty, transparent window! That’s unacceptable. A year ago, you might be right saying “Who cares about RAM, they are cheap as hell”, but check out the price they’ve grown over this year, they are expensive as hell now.

Fun fact of WPF transparent window

  • RAM usage increase as window size enlarge
  • Win32 window has no such problem

Wait, what? The larger the window is, the more RAM it consumes? Hmmmmm… this looks familiar, just like a Bitmap. For now, we don’t know how WPF handles transparent window, but the symptom shows that it’s like using the whole screen as a bitmap and the window updating itself with portion of that bitmap, making it “transparent”. What a smart move…
Back in the days when WPF was first released, low screen resolution was the main stream. Even today, most laptops still are shipping with a monitor of 1366*768 (ridiculous, right?). Let’s despise the nonsense the OEM told us and think about program running in computers with higher screen resolution.

RAM is not free, do not waste it

Obviously, costing 100+ mb of ram for showing a transparent window in 4K is unacceptable, especially compared with Win32 transparent window, which costs only 10+ mb. The gap between them makes WPF look dump.
Enough complaining, what can we do about it? I don’t want to write UI code with GDI using C++, that’s inefficient and not modern, plus, no one would abandon their beautiful, easy to maintain xaml UI code for this.

Hosting WPF content in Win32 Window

Well, indeed, no one would rewrite their UI code for just about 90mb of RAM. Compared with the work needed to rewrite C++ UI code, the RAM consumption gap seems acceptable (#smile face). But please remember, we can always host WPF content in win32 window.
Let say, we want to create a full screen notification dialog with semi-transparent background and apaque notication content in the center. To avoid the WPF ram problem, we create a semi-transparent window using win32:

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DWORD Flags1 = WS_EX_LAYERED;
DWORD Flags2 = WS_POPUP;

HWND hWnd = CreateWindowEx(Flags1,szWindowClass, szTitle, Flags2,
CW_USEDEFAULT, 0, 3840,2160, nullptr, nullptr, hInstance, nullptr);

SetLayeredWindowAttributes(hWnd, RRR, (BYTE)125, LWA_ALPHA);
ShowWindow(hWnd, nCmdShow);
UpdateWindow(hWnd);

case WM_ERASEBKGND:
RECT rect;
GetClientRect(hWnd, &rect);
FillRect((HDC)wParam, &rect, CreateSolidBrush(RGB(0, 0, 0)));
break;

By enabling C++/CLI, we can access WPF content directly

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namespace ManagedCode
{
using namespace System;
using namespace System::Windows;
using namespace System::Windows::Interop;
using namespace System::Windows::Media;

HWND GetHwnd(HWND parent, int x, int y, int width, int height) {
HwndSource^ source = gcnew HwndSource(
0, // class style
WS_VISIBLE | WS_CHILD, // style
0, // exstyle
x, y, width, height,
"hi", // NAME
IntPtr(parent) // parent window
);

UIElement^ page = gcnew ManagedContent::WPFContent();
source->RootVisual = page;
return (HWND)source->Handle.ToPointer();
}
}

and finally

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//managed content
ManagedCode::GetHwnd(hWnd, 0, 0, 200, 200);

Due to the different technologies behind WPF and GDI, more work needed to be done for the unavoidable alpha blending problem, but, you can use wpf popup to achieve your goal for short.

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