Custom button captions in .NET messagebox?

MessageBox uses a plain window that can be messed with like any other window. This has been possible in Windows for a very long time, well over 20 years already. The techniques are getting obscure though, too many friendly class wrappers that hide the native winapi and don’t expose everything you can do with it. So much so that programmers now automatically assume that this isn’t possible, as you can tell from the upvoted answers. It is the kind of programming that Petzold taught us in his seminal “Programming Windows” book. Replacing MessageBox with a custom Form or Window is actually fairly hard to do, it does non-trivial automatic layout to fit the text and supports localization without help. Although that’s exactly what you don’t seem to like 🙂

Anyhoo, the message box window is easy to find back. It is owned by the UI thread and has a special class name that makes it unique. EnumThreadWindows() enumerates the windows owned by a thread, GetClassName() lets you check the kind of window. Then just poke the text into the button with SetWindowText().

Add a new class to your project and paste the code shown below. Invoke it with code like this:

Nobugz.PatchMsgBox(New String() {"Da", "Njet"})
MsgBox("gack", MsgBoxStyle.YesNo)

Here’s the code:

Imports System.Text
Imports System.Runtime.InteropServices

Public Class Nobugz
  Private Shared mLabels() As String    '' Desired new labels
  Private Shared mLabelIndex As Integer '' Next caption to update

  Public Shared Sub PatchMsgBox(ByVal labels() As String)
    ''--- Updates message box buttons
    mLabels = labels
    Application.OpenForms(0).BeginInvoke(New FindWindowDelegate(AddressOf FindMsgBox), GetCurrentThreadId())
  End Sub

  Private Shared Sub FindMsgBox(ByVal tid As Integer)
    ''--- Enumerate the windows owned by the UI thread
    EnumThreadWindows(tid, AddressOf EnumWindow, IntPtr.Zero)
  End Sub

  Private Shared Function EnumWindow(ByVal hWnd As IntPtr, ByVal lp As IntPtr) As Boolean
    ''--- Is this the message box?
    Dim sb As New StringBuilder(256)
    GetClassName(hWnd, sb, sb.Capacity)
    If sb.ToString() <> "#32770" Then Return True
    ''--- Got it, now find the buttons
    mLabelIndex = 0
    EnumChildWindows(hWnd, AddressOf FindButtons, IntPtr.Zero)
    Return False
  End Function

  Private Shared Function FindButtons(ByVal hWnd As IntPtr, ByVal lp As IntPtr) As Boolean
    Dim sb As New StringBuilder(256)
    GetClassName(hWnd, sb, sb.Capacity)
    If sb.ToString() = "Button" And mLabelIndex <= UBound(mLabels) Then
      ''--- Got one, update text
      SetWindowText(hWnd, mLabels(mLabelIndex))
      mLabelIndex += 1
    End If
    Return True
  End Function

  ''--- P/Invoke declarations
  Private Delegate Sub FindWindowDelegate(ByVal tid As Integer)
  Private Delegate Function EnumWindowDelegate(ByVal hWnd As IntPtr, ByVal lp As IntPtr) As Boolean
  Private Declare Auto Function EnumThreadWindows Lib "user32.dll" (ByVal tid As Integer, ByVal callback As EnumWindowDelegate, ByVal lp As IntPtr) As Boolean
  Private Declare Auto Function EnumChildWindows Lib "user32.dll" (ByVal hWnd As IntPtr, ByVal callback As EnumWindowDelegate, ByVal lp As IntPtr) As Boolean
  Private Declare Auto Function GetClassName Lib "user32.dll" (ByVal hWnd As IntPtr, ByVal name As StringBuilder, ByVal maxlen As Integer) As Integer
  Private Declare Auto Function GetCurrentThreadId Lib "kernel32.dll" () As Integer
  Private Declare Auto Function SetWindowText Lib "user32.dll" (ByVal hWnd As IntPtr, ByVal text As String) As Boolean
End Class