Retrieving text from an Hwnd and releasing the allocated memory space - vb.net

Question

I am new to VB.Net. I came from vb6 and VBA so I'm still learning. I am trying to clear the memory space for when I retrieve text using my GetWinTxt() function. I am not sure if I am properly releasing the memory space or not. I have researched MSDN and a multitude of other sites including this one trying to learn about this and I find it somewhat confusing. This function will be used a lot and i don't want a memory leak. It seems to be working fine as there isn't any error codes but can someone with more knowledge please tell me if I'm doing this right? Also should I be using Marshal.DestroyStructure() before I use Marshal.FreeHGlobal()? And if so how would I use that in my code below?
Option Explicit On
Imports System.Runtime.InteropServices
Declare Auto Function SendMessage Lib "user32.dll" (ByVal hWnd As IntPtr, ByVal msg As Integer, ByVal wParam As IntPtr, ByVal lParam As IntPtr) As IntPtr
<DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)>
Shared Function FindWindow(ByVal lpClassName As String, ByVal lpWindowName As String) As IntPtr
End Function
<DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)>
Shared Function FindWindowEx(ByVal parentHandle As IntPtr, ByVal childAfter As IntPtr, ByVal lclassName As String, ByVal windowTitle As String) As IntPtr
End Function
Friend Const WM_GETTEXT = &HD
Friend Const WM_GETTEXTLENGTH = &HE
Private Sub Button5_Click(sender As Object, e As EventArgs) Handles Button5.Click
Dim Hwnd As IntPtr = FindWindow(Nothing, "Untitled - Notepad") 'notepad parent win
Dim Handle As IntPtr = FindWindowEx(Hwnd, IntPtr.Zero, "Edit", Nothing) 'notepad edit area
MsgBox(Lf.GetWinTxt(Handle))'Lf is my class name
End Sub
Friend Function GetWinTxt(hwnd As IntPtr) As String
Dim TextLen As Integer = SendMessage(hwnd, WM_GETTEXTLENGTH, 0, 0) + 1
Dim WinHandle As IntPtr = Marshal.AllocHGlobal(TextLen)
Call SendMessage(hwnd, WM_GETTEXT, TextLen, WinHandle)
Dim txt As String = Marshal.PtrToStringUni(WinHandle)
GetWinTxt = txt
Dim ByteString(TextLen) As Byte ' the next 3 lines are what i'm not sure if i'm doing it right or not
Marshal.Copy(WinHandle, ByteString, 0, TextLen)
Marshal.FreeHGlobal(txt)
End Function
EDITED FINAL CODE:
Public Function GetWinTxt(hwnd As IntPtr) As String
Dim TextLen As IntPtr = SendMessage(hwnd, WM_GETTEXTLENGTH, 0, 0) + 1
Dim f As Integer = TextLen
Winhandle = Marshal.AllocHGlobal(f)
Call SendMessage(hwnd, WM_GETTEXT, f, Winhandle)
Dim txt As String = Marshal.PtrToStringUni(Winhandle)
Return txt
Try
Dim ByteString(f) As Byte
Marshal.Copy(Winhandle, ByteString, 0, f)
Marshal.FreeHGlobal(Winhandle)
Catch ex As Exception
Return ""
Debug.Print(ex.Message)
End Try
End Function

Answer 1

The answer is no, unless you call method like Marshal. StructureToPtr().
You should use the counterpart of whatever method you used to allocate it.
Marshal. Destroy Structure will free all substructures that specify unmanaged memory block points to. But you don't have any structure in it.
In addition, you call Dim WinHandle As IntPtr = Marshal.AllocHGlobal(TextLen) Allocates memory from the unmanaged memory of the process to WinHandle, then free anther one, Marshal.FreeHGlobal(txt). This will cause memory leaks.
try to change it to Marshal.FreeHGlobal(WinHandle).