C# SCROLLINFO not working on Chrome window

Question

I am attempting to retrieve scroll information from the window handle, but I keep getting a value of 0. I am pursuing this approach because my goal is to capture a complete scrolling screenshot of the window. The intention is to enable automatic scrolling and image capture. While I have successfully implemented automatic window scrolling, I've encountered variations in the scrolling behaviour for different windows.

For instance, sending a mouse delta of 100 results in Chrome scrolling by around 70 pixels, whereas Visual Studio scrolls by approximately 110 pixels. Each window appears to exhibit distinct scrolling characteristics. Therefore, my current objective is to calculate the precise amount of pixel movement during scrolling using the scrollbar. However, I have encountered difficulties in achieving this functionality.

[Serializable, StructLayout(LayoutKind.Sequential)]
public struct SCROLLINFO
{
    public uint cbSize;
    public uint fMask;
    public int nMin;
    public int nMax;
    public uint nPage;
    public int nPos;
    public int nTrackPos;
}
public enum ScrollInfoMask : uint
{
    SIF_RANGE = 0x1,
    SIF_PAGE = 0x2,
    SIF_POS = 0x4,
    SIF_DISABLENOSCROLL = 0x8,
    SIF_TRACKPOS = 0x10,
    SIF_ALL = (SIF_RANGE | SIF_PAGE | SIF_POS | SIF_TRACKPOS),
}

public enum SBOrientation : int
{
    SB_HORZ = 0x0,
    SB_VERT = 0x1,
    SB_CTL = 0x2,
    SB_BOTH = 0x3
}

// get scrollbar info
SCROLLINFO scrollinfo = new SCROLLINFO();
scrollinfo.cbSize = (uint)Marshal.SizeOf(typeof(SCROLLINFO));
scrollinfo.fMask = (uint)ScrollInfoMask.SIF_ALL; // or SIF_PAGE | SIF_POS, etc.

bool success = GetScrollInfo(hWnd, (int)SBOrientation.SB_VERT, ref scrollinfo);
        if (success)
        {
            // Process scrollinfo here
        }
        else
        {
            // Handle error here
        }

uint errorCode = GetLastError();
        Console.WriteLine($"Failed to retrieve scroll info. Error code: {errorCode}");

[DllImport("user32.dll")]
public static extern bool GetScrollInfo(IntPtr hWnd, int fnBar, ref SCROLLINFO lpsi);

[DllImport("kernel32.dll")]
public static extern uint GetLastError();

Getting "False" from GetScrollInfo and GetLastError() returns 1447 error code

It's working on Notepad Window... but not working on Chrome, Firefox, visual studio etc

"I'm essentially looking to capture scrolling screenshots. I have two options:

Automatically scroll through the content using code and calculate the distance scrolled in pixels. Then, I'll capture that specific portion and append it to the image. Alternatively, I can scroll a predefined height. This way, if I scroll 100 pixels, for instance, I'll capture an image of exactly 100 pixels. I'm currently attempting option 1, but I haven't been successful so far."

Answer 1

The original problem lies in the fact that applications like Chrome, Firefox, and Visual Studio don't necessarily use the standard Windows scrollbar mechanism. Therefore, using the GetScrollInfo API won't work with these applications as intended. As a result, "fixing" the code in the traditional sense may not yield the desired outcome.

However, I can provide a solution based on the first recommendation: differentiating by the Window Class. The following code will attempt to use GetScrollInfo for applications that support it (like Notepad), but for known classes (like Chrome's) it will use predefined values.

Here's an updated approach:
    [DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
static extern int GetClassName(IntPtr hWnd, StringBuilder lpClassName, int nMaxCount);

public void HandleScrolling(IntPtr hWnd)
{
    StringBuilder className = new StringBuilder(256);
    GetClassName(hWnd, className, className.Capacity);

    if (className.ToString() == "Chrome_WidgetWin_1") // This is the class name for Chrome's main window as of my last update. You might need to verify.
    {
        // Handle Chrome's scrolling. You might need to tweak this.
        int pixelsScrolled = 70;
        CaptureImage(pixelsScrolled);
    }
    else
    {
        SCROLLINFO scrollinfo = new SCROLLINFO();
        scrollinfo.cbSize = (uint)Marshal.SizeOf(typeof(SCROLLINFO));
        scrollinfo.fMask = (uint)ScrollInfoMask.SIF_ALL;

        bool success = GetScrollInfo(hWnd, (int)SBOrientation.SB_VERT, ref scrollinfo);
        if (success)
        {
            int pixelsScrolled = scrollinfo.nPos; // This is a simplistic approach. You might need to calculate the difference from the previous nPos value.
            CaptureImage(pixelsScrolled);
        }
        else
        {
            uint errorCode = GetLastError();
            Console.WriteLine($"Failed to retrieve scroll info for window class {className}. Error code: {errorCode}");
        }
    }
}

public void CaptureImage(int pixelsScrolled)
{
    // Your logic to capture an image of the specified height.
}

In this approach, we first check the class name of the window. If it's a known class like Chrome, we use predefined scrolling values. For other windows, we attempt to use the GetScrollInfo API.

Remember, this method is still somewhat fragile because it relies on known window class names and predefined scrolling values. You'll need to maintain and update this list as needed.

Answer 2

There are no scrollbars...

These days you rarely see modern applications with native scrollbars. Visual Studio has one single top-level native window handle (a HWND). The top-level window could theoretically have a scrollbar, but usually will not.

...just pixels!

Now, when you see those scrollbars in, what appears to be child windows, in e.g. Visual Studio and Chrome, chances are that those are just rendered pixels. I.e. There is no native scrollbar to look for.

Since we know that Visual Studio is a WPF application, we can get hold of scrollbar information using some WPF related tricks. Obviously, such tricks will not work for non-WPF applications (like Chrome). For Chrome there might be some way to read the DOM outside the browser, but expect it to be non-trivial.

Answer 3

Make sure you initialize the SCROLLINFO structure properly before using it.

SCROLLINFO scrollinfo = new SCROLLINFO();
scrollinfo.cbSize = (uint)Marshal.SizeOf(typeof(SCROLLINFO));
scrollinfo.fMask = SIF_ALL; // or SIF_PAGE | SIF_POS, etc.

And handle return value of GetScrollInfo: The GetScrollInfo function returns a boolean indicating success or failure. Make sure you're checking this return value to ensure that the scroll information is indeed being retrieved.

bool success = GetScrollInfo(windowHandle, SB_HORZ, ref scrollinfo);
if (success)
{
    // Process scrollinfo here
}
else
{
    // Handle error here
}

Use the Right Scroll Bar Constant: The fnBar parameter in the GetScrollInfo function indicates whether you're querying the horizontal or vertical scrollbar. Make sure you're using the correct constant values: SB_HORZ for horizontal and SB_VERT for vertical scrollbars.