用WinAPI实现一个图片查看器：一个打开按钮，点击后打开一个打开文件对话框，打开选中的png文件显示在屏幕上，并可以用鼠标滚轮调整大小

你可以使用WinAPI来实现一个图片查看器，以下是一个基本的示例代码：

#include <windows.h>

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam);

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow) {
    // 创建窗口类
    WNDCLASS wc = { 0 };
    wc.lpfnWndProc = WndProc;
    wc.hInstance = hInstance;
    wc.hCursor = LoadCursor(NULL, IDC_ARROW);
    wc.lpszClassName = "ImageViewer";

    // 注册窗口类
    RegisterClass(&wc);

    // 创建窗口
    HWND hwnd = CreateWindow("ImageViewer", "Image Viewer", WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, 800, 600, NULL, NULL, hInstance, NULL);

    // 显示窗口
    ShowWindow(hwnd, nCmdShow);
    UpdateWindow(hwnd);

    // 消息循环
    MSG msg;
    while (GetMessage(&msg, NULL, 0, 0)) {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    return msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam) {
    static HBITMAP hBitmap = NULL;

    switch (msg) {
        case WM_CREATE: {
            // 创建打开按钮
            CreateWindow("BUTTON", "Open", WS_VISIBLE | WS_CHILD, 10, 10, 80, 30, hwnd, (HMENU)1, NULL, NULL);
            break;
        }
        case WM_COMMAND: {
            if (LOWORD(wParam) == 1) {
                // 打开文件对话框
                char szFile[MAX_PATH] = { 0 };
                OPENFILENAME ofn = { 0 };
                ofn.lStructSize = sizeof(ofn);
                ofn.hwndOwner = hwnd;
                ofn.lpstrFile = szFile;
                ofn.nMaxFile = sizeof(szFile);
                ofn.lpstrFilter = "PNG Files\0*.png\0All Files\0*.*\0";
                ofn.nFilterIndex = 1;
                ofn.Flags = OFN_PATHMUSTEXIST | OFN_FILEMUSTEXIST;

                if (GetOpenFileName(&ofn)) {
                    // 加载图片
                    hBitmap = (HBITMAP)LoadImage(NULL, ofn.lpstrFile, IMAGE_BITMAP, 0, 0, LR_LOADFROMFILE);

                    // 刷新窗口
                    InvalidateRect(hwnd, NULL, TRUE);
                }
            }
            break;
        }
        case WM_PAINT: {
            PAINTSTRUCT ps;
            HDC hdc = BeginPaint(hwnd, &ps);

            // 绘制图片
            if (hBitmap) {
                HDC hdcMem = CreateCompatibleDC(hdc);
                HBITMAP hOldBitmap = (HBITMAP)SelectObject(hdcMem, hBitmap);

                BITMAP bmp;
                GetObject(hBitmap, sizeof(BITMAP), &bmp);

                int width = bmp.bmWidth;
                int height = bmp.bmHeight;

                // 调整图片大小以适应窗口
                RECT clientRect;
                GetClientRect(hwnd, &clientRect);
                float aspectRatio = (float)width / height;
                int newWidth = clientRect.right - clientRect.left;
                int newHeight = (int)(newWidth / aspectRatio);
                if (newHeight > clientRect.bottom - clientRect.top) {
                    newHeight = clientRect.bottom - clientRect.top;
                    newWidth = (int)(newHeight * aspectRatio);
                }

                // 居中绘制
                int x = (clientRect.right - clientRect.left - newWidth) / 2;
                int y = (clientRect.bottom - clientRect.top - newHeight) / 2;

                StretchBlt(hdc, x, y, newWidth, newHeight, hdcMem, 0, 0, width, height, SRCCOPY);

                SelectObject(hdcMem, hOldBitmap);
                DeleteDC(hdcMem);
            }

            EndPaint(hwnd, &ps);
            break;
        }
        case WM_MOUSEWHEEL: {
            // 使用鼠标滚轮调整图片大小
            if (hBitmap) {
                int delta = GET_WHEEL_DELTA_WPARAM(wParam);
                float scaleFactor = 1.1f;

                if (delta > 0) {
                    // 向上滚动，放大图片
                    int newWidth = (int)(GetWidth(hBitmap) * scaleFactor);
                    int newHeight = (int)(GetHeight(hBitmap) * scaleFactor);
                    hBitmap = (HBITMAP)CopyImage(hBitmap, IMAGE_BITMAP, newWidth, newHeight, LR_COPYRETURNORG);
                } else {
                    // 向下滚动，缩小图片
                    int newWidth = (int)(GetWidth(hBitmap) / scaleFactor);
                    int newHeight = (int)(GetHeight(hBitmap) / scaleFactor);
                    hBitmap = (HBITMAP)CopyImage(hBitmap, IMAGE_BITMAP, newWidth, newHeight, LR_COPYRETURNORG);
                }

                // 刷新窗口
                InvalidateRect(hwnd, NULL, TRUE);
            }
            break;
        }
        case WM_DESTROY: {
            // 释放资源
            if (hBitmap) {
                DeleteObject(hBitmap);
            }
            PostQuitMessage(0);
            break;
        }
        default: {
            return DefWindowProc(hwnd, msg, wParam, lParam);
        }
    }

    return 0;
}

这段代码创建了一个简单的窗口应用程序，包含一个"Open"按钮和一个用于显示图片的窗口。点击"Open"按钮会打开一个文件对话框，选择一个PNG文件后，该文件将会显示在窗口中。你可以使用鼠标滚轮来调整图片的大小。注意，这段代码并没有进行错误处理和内存管理，你可以根据自己的需要进行相应的改进。