网上很多说用hook还有写乱七八糟一堆代码。我这唯一找到一个很好的方案：

xaml文件加入这句代码：完美解决

private void ButtonMax_Click(object sender, RoutedEventArgs e)
        {
            if (this.WindowState == WindowState.Maximized)
            {
                this.WindowState = WindowState.Normal;
            }
            else if (this.WindowState == WindowState.Normal)
            {
                this.WindowState = WindowState.Maximized;
                //代码在这
                this.MaxHeight = SystemParameters.MaximizedPrimaryScreenHeight;
                this.MaxWidth = SystemParameters.MaximizedPrimaryScreenWidth;
            }
        }

或者在xaml文件里加上这句

<Window x:Class="Working.Ui.WindowManagement"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="" Height="960" Width="1280" WindowStyle="None" MaxWidth="{StaticResource {x:Static SystemParameters.MaximizedPrimaryScreenWidthKey}}" MaxHeight="{StaticResource {x:Static SystemParameters.MaximizedPrimaryScreenHeightKey}}">