C# Signature:

[DllImport("hhctrl.ocx", SetLastError=true)]
static extern IntPtr HtmlHelp( IntPtr hWndCaller, string helpFile, HTMLHelpCommand command, int data );

User-Defined Types:

    [Flags()]
    public enum HTMLHelpCommand : uint
    {
        HH_DISPLAY_TOPIC = 0,
        HH_DISPLAY_TOC = 1,
        HH_DISPLAY_INDEX = 2,
        HH_DISPLAY_SEARCH = 3,
        HH_HELP_CONTEXT = 0x000F,
        HH_CLOSE_ALL = 0x0012
    }

VB Signature:

<DllImport("hhctrl.ocx", EntryPoint:="HtmlHelp", CharSet:=CharSet.Auto)> _
Function HTMLHelp( _
     ByVal hWndCaller As IntPtr, ByVal pszFile As String, _
     ByVal uCommand As Integer, ByVal dwData As Integer) As Integer
End Function

User-Defined Types:

Public Enum HTMLHelpCommand
     HH_DISPLAY_TOPIC = 0
     HH_DISPLAY_TOC = 1
     HH_DISPLAY_INDEX = 2
     HH_DISPLAY_SEARCH = 3
     HH_HELP_CONTEXT = &HF
     HH_CLOSE_ALL = &H12
End Enum

Notes:

None.

Tips & Tricks:

Article "How to use the unmanaged HTML Help API from a managed Visual C# application" on support.microsoft.com

Sample Code:

HTMLHelp(Nothing, filePath, HTMLHelpCommand.HH_HELP_CONTEXT, contextID)

or

HTMLHelp(Nothing, filePath, HTMLHelpCommand.HH_DISPLAY_TOC, 0)

Alternative Managed API:

System.Windows.Forms.Help.ShowHelp()