[WPF][ウインドウを最大化した時のサイズと位置を制限する Behavior とその使用サンプル

MainWindow.xaml

<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
        xmlns:ei="http://schemas.microsoft.com/expression/2010/interactions"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" 
        Height="450" 
        Width="800">

    <i:Interaction.Behaviors>
        <local:MaximizedWindowBehavior MaximizedBounds="200,500,1920,1080" />
    </i:Interaction.Behaviors>

    <Grid>
    </Grid>
</Window>

MaximizedWindowBehavior.cs

using System;
using System.ComponentModel;
using System.Runtime.InteropServices;
using System.Windows;
using System.Windows.Interactivity;
using System.Windows.Interop;

namespace WpfApp1 {
    class MaximizedWindowBehavior: Behavior<Window> {

        #region MaximizedBounds Property

        public Rect MaximizedBounds {
            get => (Rect)this.GetValue( MaximizedBoundsProperty );
            set => this.SetValue( MaximizedBoundsProperty, value );
        }

        public static readonly DependencyProperty MaximizedBoundsProperty =
            DependencyProperty.Register( nameof( MaximizedBounds ), typeof( Rect ), typeof( MaximizedWindowBehavior ), new PropertyMetadata( Rect.Empty ) );

        #endregion

        public MaximizedWindowBehavior() { }

        protected override void OnAttached() {
            base.OnAttached();
            this.AssociatedObject.Loaded += this.WindowLoaded;
            this.AssociatedObject.Closing += this.WindowClosing;
        }

        protected override void OnDetaching() {
            this.AssociatedObject.Loaded -= this.WindowLoaded;
            this.AssociatedObject.Closing -= this.WindowClosing;
            base.OnDetaching();
        }

        private void WindowLoaded( object sender, RoutedEventArgs e ) {
            var hwnd = new WindowInteropHelper( this.AssociatedObject ).Handle;
            var source = HwndSource.FromHwnd( hwnd );
            source.AddHook( this.WndProc );
        }

        private void WindowClosing( object sender, CancelEventArgs e ) {
            var hwnd = new WindowInteropHelper( this.AssociatedObject ).Handle;
            var source = HwndSource.FromHwnd( hwnd );
            source.RemoveHook( this.WndProc );
        }

        #region Win32

        private struct POINT {
            public int x;
            public int y;
        }

#pragma warning disable CS0649
        private struct MINMAXINFO {
            public POINT ptReserved;
            public POINT ptMaxSize;         // 最大化フォームのサイズ
            public POINT ptMaxPosition;     // 最大化フォームの位置
            public POINT ptMinTrackSize;    // フォームの最大サイズ
            public POINT ptMaxTrackSize;    // フォームの最小サイズ
        }
#pragma warning restore CS0649

        private const int WM_GETMINMAXINFO = 0x0024;

        #endregion

        private IntPtr WndProc( IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled ) {
            if( msg == WM_GETMINMAXINFO ) {
                handled = true;

                var info = Marshal.PtrToStructure<MINMAXINFO>( lParam );
                info.ptMaxPosition.x = (int)this.MaximizedBounds.Left;
                info.ptMaxPosition.y = (int)this.MaximizedBounds.Top;
                if( 0 < this.MaximizedBounds.Width )
                    info.ptMaxSize.x = (int)this.MaximizedBounds.Width;
                if( 0 < this.MaximizedBounds.Height )
                    info.ptMaxSize.y = (int)this.MaximizedBounds.Height;
                Marshal.StructureToPtr( info, lParam, true );

                return IntPtr.Zero;
            }
            return IntPtr.Zero;
        }

    }
}