通过textbox的句柄得到并且修改其值

[zzmsyt]

我想获取浏览器中网址的信息，然后再网址后面继续添加一些内容！
我利用spy++获取了浏览器中的textbox的句柄值，现在要获取其中的值，看网上说GetWindowTxt不行，得利用sendmessage这个函数，没怎么弄过api求解答！
我的代码
[DllImport("User32.dll", EntryPoint = "SendMessage")]
private static extern int SendMessage(IntPtr hWnd, int Msg, IntPtr wParam, string lParam);
const int WM_GETTEXT = 0x000D;
const int WM_SETTEXT = 0x000C;


int a = int.Parse("000203AA", NumberStyles.HexNumber);//00203AA是spy++获取的
IntPtr p=(IntPtr)a;
StringBuilder st= new StringBuilder(256);
SendMessage(p, WM_GETTEXT,IntPtr.Zero, st.ToString());
MessageBox.Show(st.ToString());

[xingyuebuyu]

  [DllImport("User32.dll", EntryPoint = "SendMessage")]
  private static extern int SendMessage(IntPtr hWnd, int Msg, IntPtr wParam, StringBuilder lParam);
  const int WM_GETTEXT = 0x000D;
  const int WM_SETTEXT = 0x000C;

int a = int.Parse("000203AA", NumberStyles.HexNumber);//00203AA是spy++获取的
 IntPtr p=(IntPtr)a;
 StringBuilder st= new StringBuilder(256);
 SendMessage(p, WM_GETTEXT,IntPtr.Zero, st);
 MessageBox.Show(st.ToString());

传StringBuilder才能获取到值

[zzmsyt]

改成stringbuilder还是空，难道浏览器里的取不到值？

[zzmsyt]

我又试试别的，那个都取不到值 无语。。。。