Window2 wnd = new Window2();
wnd.Show();

wnd.Owner = this; // 设置子窗体的父窗体为当前窗体

// 添加API引用
[System.Runtime.InteropServices.DllImport("user32.dll", EntryPoint = "SetParent")]
public extern static IntPtr SetParent(IntPtr childPtr, IntPtr parentPtr);

// 实例化子窗体
Window2 wnd = new Window2();
wnd.Show(); 

IntPtr parentHandle = new WindowInteropHelper(this).Handle; // 获得该窗体的句柄
IntPtr childHandle = new WindowInteropHelper(wnd).Handle;
SetParent(childHandle, parentHandle); // 设置MDI窗体

private const int GWL_STYLE = -16; // GWL_STYLE 指的是那些旧的窗口属性。相对于GWL_EXSTYLEGWL扩展属性而言的
private const long WS_CHILD = 0x40000000L;  // Popup风格

[DllImport("user32.dll")]
private extern static int SetWindowLong(IntPtr hwnd, int oldStyle, long newStyle); //newStyle采用long类型有时候会报错，建议采用int类型
[DllImport("user32.dll")]
private extern static int GetWindowLong(IntPtr hwnd, int oldStyle);

private void Window_Loaded_1(object sender, RoutedEventArgs e)
{
   // 注意:该段代码不能写在构造函数中，因为窗体未加载，没有句柄
    IntPtr handle = new WindowInteropHelper(this).Handle;
    int style = GetWindowLong(handle, GWL_STYLE);
    SetWindowLong(handle, GWL_STYLE, style | WS_CHILD);
}

Window activeWindow=App.Current.Windows.OfType<Window>().SingleOrDefault(x=>x.IsActive)

// 获得活动窗体的句柄
[DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
private extern IntPtr GetForegroundWindow();
// 获得根据窗体句柄窗体的名称
[DllImport("User32.dll")]
private extern int GetWindowText(IntPtr handle, StringBuilder text, int maxLen);

StringBuilder text = new StringBuilder(512); // 定义接受窗体名称的变量
IntPtr myPtr = GetForegroundWindow();
int i = GetWindowText(myPtr, text, 512);