C# Signature:

[DllImport("user32.dll")]
static extern bool WinHelp(IntPtr hWndMain, string lpszHelp, uint uCommand,
   uint dwData);

VB.NET Signature:

<DllImport("user32.dll", SetLastError:=True, Charset:=Charset.Ansi)> _
Public Function bool WinHelp(hWndMain As IntPtr, _
    <MarshalAs(UnmanagedType.LPStr)>lpszHelp As String, _
    uCommand As UInteger, _
    dwData As UInteger) As <MarshalAs(UnmanagedType.Bool)> Boolean
End function

User-Defined Types:

[Enums.WinHelpCommands]

Notes:

Here are the constants defined in Winver.h:

    // Commands to pass to WinHelp()
    const uint HELP_CONTEXT      = 0x0001;  /* Display topic in ulTopic */
    const uint HELP_QUIT         = 0x0002;  /* Terminate help */
    const uint HELP_INDEX    = 0x0003;  /* Display index */
    const uint HELP_CONTENTS     = 0x0003;
    const uint HELP_HELPONHELP   = 0x0004;  /* Display help on using help */
    const uint HELP_SETINDEX     = 0x0005;  /* Set current Index for multi index help */
    const uint HELP_SETCONTENTS  = 0x0005;
    const uint HELP_CONTEXTPOPUP = 0x0008;
    const uint HELP_FORCEFILE    = 0x0009;
    const uint HELP_KEY      = 0x0101; /* Display topic for keyword in offabData */
    const uint HELP_COMMAND      = 0x0102;
    const uint HELP_PARTIALKEY   = 0x0105;
    const uint HELP_MULTIKEY     = 0x0201;
    const uint HELP_SETWINPOS    = 0x0203;
    const uint HELP_CONTEXTMENU  = 0x000a;
    const uint HELP_FINDER       = 0x000b;
    const uint HELP_WM_HELP      = 0x000c;
    const uint HELP_SETPOPUP_POS = 0x000d;

    const uint HELP_TCARD          = 0x8000;
    const uint HELP_TCARD_DATA     = 0x0010;
    const uint HELP_TCARD_OTHER_CALLER = 0x0011;

    const uint IDH_NO_HELP         = 28440;
    const uint IDH_MISSING_CONTEXT     = 28441; // Control doesn't have matching help context
    const uint IDH_GENERIC_HELP_BUTTON = 28442; // Property sheet help button
    const uint IDH_OK          = 28443;
    const uint IDH_CANCEL          = 28444;
    const uint IDH_HELP        = 28445;

Tips & Tricks:

Please add some!

Sample Code:

    [DllImport("user32.dll")]
    static extern IntPtr FindWindow(string lpClassName, string lpWindowName);

    [DllImport("user32.dll")]
    static extern bool WinHelp(IntPtr hWndMain, string lpszHelp, uint uCommand, uint dwData);

    const uint YOUR_CONTEXT_SENSITIVE_HELP_ID = 32862;
    string lpszClass = "WindowsForms10.Window.8.app3";
    string lpszWindow = "Your Window Title";

    IntPtr ParentHWnd = new IntPtr(0);
    ParentHWnd = FindWindow(lpszClass, lpszWindow);

    if (!ParentHWnd.Equals(IntPtr.Zero))
    {
        bool ret = WinHelp(ParentHWnd, "Your help file.hlp", HELP_CONTEXT, YOUR_CONTEXT_SENSITIVE_HELP_ID);
    }

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