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SendMessage (user32)

C# Signature:

[DllImport("user32.dll", SetLastError=true, CharSet=CharSet.Auto)]
public static extern IntPtr SendMessageTimeout(
    IntPtr hWnd,
    uint Msg,
    UIntPtr wParam,
    IntPtr lParam,
    SendMessageTimeoutFlags fuFlags,
    uint uTimeout,
    out UIntPtr lpdwResult);

[DllImport("user32.dll", SetLastError=true, CharSet=CharSet.Auto)]
public static extern IntPtr SendMessageTimeout(
    IntPtr windowHandle,
    uint Msg,
    IntPtr wParam,
    IntPtr lParam,
    SendMessageTimeoutFlags flags,
    uint timeout,
    out IntPtr result);

/* Version specifically setup for use with WM_GETTEXT message */

[DllImport("user32.dll", EntryPoint = "SendMessageTimeout", SetLastError=true, CharSet=CharSet.Auto)]
public static extern uint SendMessageTimeoutText(
    IntPtr hWnd,
    int Msg,              // Use WM_GETTEXT
    int countOfChars,
    StringBuilder text,
    SendMessageTimeoutFlags flags,
    uint uTImeoutj,
    out IntPtr result);

/* Version for a message which returns an int, such as WM_GETTEXTLENGTH. */

[DllImport("user32.dll", EntryPoint = "SendMessageTimeout", CharSet = CharSet.Auto)]
public static extern int SendMessageTimeout(
    IntPtr hwnd,
    uint Msg,
    int wParam,
    int lParam,
    uint fuFlags,
    uint uTimeout,
    out int lpdwResult);

User-Defined Types:


VB Signature:

<DllImport("user32.dll", SetLastError:=True)> _
Public Sub SendMessageTimeout(ByVal windowHandle As IntPtr,ByVal Msg As Integer,ByVal wParam As IntPtr,ByVal lParam As IntPtr,ByVal flags As SendMessageTimeoutFlags,ByVal timeout As Integer,ByRef result As IntPtr)
End Sub

<DllImport("user32.dll", SetLastError:=True)> _
Public Shared Function SendMessageTimeout(ByVal windowHandle As IntPtr, ByVal Msg As Integer, ByVal wParam As IntPtr, ByVal lParam As IntPtr, ByVal flags As SendMessageTimeoutFlags, ByVal timeout As Integer, ByRef result As IntPtr) As IntPtr
End Function



Tips & Tricks:

Used when propagating environment variables, for instance, updating the Path, without requiring a reboot: see also

Please add some!

Sample Code:

C# Sample Code

//Register the message
lMsg = Win32.RegisterWindowMessage("WM_HTML_GETOBJECT");
//Get the object
Win32.SendMessageTimeout(windowHandle, lMsg, IntPtr.Zero, IntPtr.Zero, SendMessageTimeoutFlags.SMTO_ABORT_IF_HUNG, 1000, out lRes);
if(lRes != IntPtr.Zero)
    //Get the object from lRes
    htmlDoc= (mshtml.IHTMLDocument)Win32.ObjectFromLresult(lRes, IID_IHTMLDocument, IntPtr.Zero);
    return htmlDoc;


const int NoResponse = -1;
const uint WM_GETTEXTLENGTH = 0x000E;
static int GetWindowTextLength(IntPtr hwnd) {
    int length;
    int result = SendMessageTimeout(hwnd, WM_GETTEXTLENGTH, 0, 0, 0X2C, 5, out length); //0X2C = SMTO_ABORTIFHUNG | SMTO_BLOCK | SMTO_NOTIMEOUTIFNOTHUNG | SMTO_ERRORONEXIT
    if (result == 0) return NoResponse; else return length;

Alternative Managed API:

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