程序代码：

Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare Function FindWindowEx Lib "user32" Alias "FindWindowExA" (ByVal hWnd1 As Long, ByVal hWnd2 As Long, ByVal lpsz1 As String, ByVal lpsz2 As String) As Long
Private Declare Function GetWindowText Lib "user32" Alias "GetWindowTextA" (ByVal hwnd As Long, ByVal lpString As String, ByVal cch As Long) As Long
Private Declare Function GetWindowTextLength Lib "user32" Alias "GetWindowTextLengthA" (ByVal hwnd As Long) As Long
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long

Private Declare Sub RtlMoveMemory Lib "KERNEL32" (lpvDest As Any, lpvSource As Any, ByVal cbCopy As Long)
Private Const WM_GETTEXT = &HD
Private Const WM_GETTEXTLENGTH = &HE

程序代码：

Dim lHwnd As Long
Dim lChildHwnd As Long
Dim strRet As String
Dim n As Long
Dim bArr() As Byte, bArr2() As Byte

lHwnd = FindWindow(vbNullString, "计算器")
If lHwnd > 0 Then
    lChildHwnd = FindWindowEx(lHwnd, 0&, "Edit", vbNullString)
    If lChildHwnd > 0 Then
        ' 获取文本长度
        n = SendMessage(lChildHwnd, WM_GETTEXTLENGTH, 0, 0)
        If n > 0 Then
            ' 设置缓冲区
            ReDim bArr(n + 1) As Byte
            ReDim bArr2(n - 1) As Byte
            Call RtlMoveMemory(bArr(0), n, 2)
            '发送 WM_GETTEXT 消息
            Call SendMessage(lChildHwnd, WM_GETTEXT, n + 1, bArr(0))
            Call RtlMoveMemory(bArr2(0), bArr(0), n)
            ' 得到文本
            strRet = StrConv(bArr2, vbUnicode)
            
            '//! 对于计算器的文本框不能直接用GetWindowText.
            'n = GetWindowTextLength(lChildHwnd)
            'strRet = String(n, "")
            'call GetWindowText(lChildHwnd, strRet, n)
            Debug.Print "text=", strRet
        End If
    End If
End If