回答編集履歴

  この手法は裏技？だと思うので、アプリごとに試行錯誤が必要になると思います。

  APIの呼び出し順やパラメータがシビアで、ちょっとの違いでできたりできなかったりします。

 

  <Window

  x:Class="Questions243916.MainWindow"

  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

  xmlns:forms="clr-namespace:System.Windows.Forms;assembly=System.Windows.Forms"

  Width="800"

  Height="450">

  <Grid>

  <Grid.ColumnDefinitions>

  <ColumnDefinition />

  <ColumnDefinition Width="Auto" />

  <ColumnDefinition />

  </Grid.ColumnDefinitions>

  <WindowsFormsHost>

  <forms:Control x:Name="notepad" />

  </WindowsFormsHost>

  <GridSplitter

  Grid.Column="1"

  Width="8"

  HorizontalAlignment="Stretch"

  Background="DarkGray" />

  <WindowsFormsHost Grid.Column="2">

  <forms:Control x:Name="paint" />

  </WindowsFormsHost>

  </Grid>

  </Window>

  ```

 

  using System;

  using System.Diagnostics;

  using System.Runtime.InteropServices;

  using System.Threading;

  using System.Windows;

 

  namespace Questions243916

  {

  public partial class MainWindow : Window

  {

  [DllImport("user32")] private static extern int GetWindowLong(IntPtr hWnd, int nIndex);

  [DllImport("user32")] private static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);

  [DllImport("user32")] private static extern IntPtr SetParent(IntPtr hWndChild, IntPtr hWndNewParent);

  [DllImport("user32")] private static extern int MoveWindow(IntPtr hwnd, int x, int y, int nWidth, int nHeight, int bRepaint);

 

  private const int GWL_STYLE = -16;

  private const int WS_THICKFRAME = 0x00040000;

  private const int WS_CAPTION = 0x00C00000;

  private const int WS_CHILD = 0x40000000;

 

 

  public MainWindow()

  {

  InitializeComponent();

 

  {

  var p = Process.Start(new ProcessStartInfo

  {

  FileName = "notepad",

 

  // HiddenとしたいがMainWindowHandleが0になる

  // FindWindowで探す必要あり 面倒なのでMinimized

  //WindowStyle = ProcessWindowStyle.Hidden,

  WindowStyle = ProcessWindowStyle.Minimized,

  });

  p.WaitForInputIdle();

 

  var style = GetWindowLong(p.MainWindowHandle, GWL_STYLE);

  style = style & ~WS_CAPTION & ~WS_THICKFRAME;

  //style |= WS_CHILD; // メニュー有無

  SetWindowLong(p.MainWindowHandle, GWL_STYLE, style);

 

  SetParent(p.MainWindowHandle, notepad.Handle);

 

  notepad.SizeChanged += (s, e)

  => MoveWindow(p.MainWindowHandle, 0, 0, notepad.Width, notepad.Height, 1);

  }

 

  {

  var p = Process.Start(new ProcessStartInfo

  {

  FileName = "mspaint",

  WindowStyle = ProcessWindowStyle.Minimized,

  });

  p.WaitForInputIdle();

  Thread.Sleep(500); // ウェイトがないと入らなかった

 

  var style = GetWindowLong(p.MainWindowHandle, GWL_STYLE);

  style = style & ~WS_CAPTION & ~WS_THICKFRAME;

  SetWindowLong(p.MainWindowHandle, GWL_STYLE, style);

 

  SetParent(p.MainWindowHandle, paint.Handle);

 

  // なんかずれる それっぽくなったが根拠なし

  var offset = (int)SystemParameters.MenuHeight

  + (int)SystemParameters.ResizeFrameHorizontalBorderHeight * 2;

  paint.SizeChanged += (s, e)

  => MoveWindow(p.MainWindowHandle, 0, -offset, paint.Width, paint.Height + offset, 1);

  }

  }

  }

  }

  ```

 

  ---

 

  メモ帳とペイントを入れるだけでかなり苦労しました^^;