Image.NineGrid for WPF

generic.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:local="clr-namespace:NineGridImage">
    
    <Style TargetType="Rectangle">
        <Setter Property="SnapsToDevicePixels" Value="True"/>
    </Style>

    <Style TargetType="{x:Type local:NineGridImage}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type local:NineGridImage}">
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="*"/>
                            <ColumnDefinition Width="Auto"/>
                        </Grid.ColumnDefinitions>
                        <Grid.RowDefinitions>
                            <RowDefinition Height="Auto"/>
                            <RowDefinition Height="*"/>
                            <RowDefinition Height="Auto"/>
                        </Grid.RowDefinitions>

                        <Rectangle x:Name="PART_topleft">
                            <Rectangle.Fill>
                                <ImageBrush ImageSource="{Binding Path=Source, RelativeSource={RelativeSource AncestorType=local:NineGridImage}}" 
                                            x:Name="brush1"/>
                            </Rectangle.Fill>
                        </Rectangle>
                        <Rectangle Grid.Column="1" Grid.Row="0" x:Name="PART_topmid">
                            <Rectangle.Fill>
                                <ImageBrush ImageSource="{Binding Path=Source, RelativeSource={RelativeSource AncestorType=local:NineGridImage}}" 
                                            x:Name="brush2"/>
                            </Rectangle.Fill>
                        </Rectangle>
                        <Rectangle Grid.Column="2" Grid.Row="0">
                            <Rectangle.Fill>
                                <ImageBrush ImageSource="{Binding Path=Source, RelativeSource={RelativeSource AncestorType=local:NineGridImage}}" 
                                            x:Name="brush3"/>
                            </Rectangle.Fill>
                        </Rectangle>

                        <Rectangle Grid.Row="1" x:Name="PART_midleft">
                            <Rectangle.Fill>
                                <ImageBrush ImageSource="{Binding Path=Source, RelativeSource={RelativeSource AncestorType=local:NineGridImage}}" 
                                            x:Name="brush4"/>
                            </Rectangle.Fill>
                        </Rectangle>
                        <Rectangle Grid.Column="1" Grid.Row="1">
                            <Rectangle.Fill>
                                <ImageBrush ImageSource="{Binding Path=Source, RelativeSource={RelativeSource AncestorType=local:NineGridImage}}" 
                                            x:Name="brush5"/>
                            </Rectangle.Fill>
                        </Rectangle>
                        <Rectangle Grid.Column="2" Grid.Row="1">
                            <Rectangle.Fill>
                                <ImageBrush ImageSource="{Binding Path=Source, RelativeSource={RelativeSource AncestorType=local:NineGridImage}}" 
                                            x:Name="brush6"/>
                            </Rectangle.Fill>
                        </Rectangle>

                        <Rectangle Grid.Row="2">
                            <Rectangle.Fill>
                                <ImageBrush ImageSource="{Binding Path=Source, RelativeSource={RelativeSource AncestorType=local:NineGridImage}}" 
                                            x:Name="brush7"/>
                            </Rectangle.Fill>
                        </Rectangle>
                        <Rectangle Grid.Column="1" Grid.Row="2">
                            <Rectangle.Fill>
                                <ImageBrush ImageSource="{Binding Path=Source, RelativeSource={RelativeSource AncestorType=local:NineGridImage}}" 
                                            x:Name="brush8"/>
                            </Rectangle.Fill>
                        </Rectangle>
                        <Rectangle Grid.Column="2" Grid.Row="2" x:Name="PART_bottomright">
                            <Rectangle.Fill>
                                <ImageBrush ImageSource="{Binding Path=Source, RelativeSource={RelativeSource AncestorType=local:NineGridImage}}" 
                                            x:Name="brush9"/>
                            </Rectangle.Fill>
                        </Rectangle>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
    
</ResourceDictionary>

NineGridImage.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;

namespace NineGridImage
{
    [TemplatePart(Name = "brush1", Type = typeof(ImageBrush))]
    [TemplatePart(Name = "brush2", Type = typeof(ImageBrush))]
    [TemplatePart(Name = "brush3", Type = typeof(ImageBrush))]
    [TemplatePart(Name = "brush4", Type = typeof(ImageBrush))]
    [TemplatePart(Name = "brush5", Type = typeof(ImageBrush))]
    [TemplatePart(Name = "brush6", Type = typeof(ImageBrush))]
    [TemplatePart(Name = "brush7", Type = typeof(ImageBrush))]
    [TemplatePart(Name = "brush8", Type = typeof(ImageBrush))]
    [TemplatePart(Name = "brush9", Type = typeof(ImageBrush))]
    [TemplatePart(Name = PART_topleft, Type = typeof(Rectangle))]
    [TemplatePart(Name = PART_midleft, Type = typeof(Rectangle))]
    [TemplatePart(Name = PART_topmid, Type = typeof(Rectangle))]
    [TemplatePart(Name = PART_bottomright, Type = typeof(Rectangle))]
    public class NineGridImage : Control
    {
        private const string PART_topleft = "PART_topleft";
        private const string PART_midleft = "PART_midleft";
        private const string PART_topmid = "PART_topmid";
        private const string PART_bottomright = "PART_bottomright";

        public static readonly DependencyProperty SourceProperty =
            DependencyProperty.Register("Source", typeof(ImageSource), typeof(NineGridImage), new UIPropertyMetadata(null, PropertyChanged));

        public static readonly DependencyProperty OffsetsProperty =
            DependencyProperty.Register("Offsets", typeof(Thickness), typeof(NineGridImage), new PropertyMetadata(new Thickness(), PropertyChanged));

        private ImageBrush[] image = new ImageBrush[9];
        private Rectangle topleft, midleft, topmid, bottomright;

        static NineGridImage()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(NineGridImage), new FrameworkPropertyMetadata(typeof(NineGridImage)));
        }

        public NineGridImage()
        {
            this.SizeChanged += NineGridImage_SizeChanged;
        }

        void NineGridImage_SizeChanged(object sender, SizeChangedEventArgs e)
        {
            this.UpdateNines();
            this.UpdateSizes();
        }

        public ImageSource Source
        {
            get { return (ImageSource)GetValue(SourceProperty); }
            set { SetValue(SourceProperty, value); }
        }

        public Thickness Offsets
        {
            get { return (Thickness)GetValue(OffsetsProperty); }
            [TypeConverter(typeof(ThicknessConverter))]
            set { SetValue(OffsetsProperty, value); }
        }

        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();

            for (int i = 0; i < 9; i++)
            {
                image[i] = GetTemplateChild("brush" + (i + 1)) as ImageBrush;
            }
            this.topleft = GetTemplateChild(PART_topleft) as Rectangle;
            this.topmid = GetTemplateChild(PART_topmid) as Rectangle;
            this.midleft = GetTemplateChild(PART_midleft) as Rectangle;
            this.bottomright = GetTemplateChild(PART_bottomright) as Rectangle;

            this.UpdateSizes();
            this.UpdateNines();
        }

        public static void PropertyChanged(DependencyObject sender, DependencyPropertyChangedEventArgs a)
        {
            var ng = sender as NineGridImage;
            ng.MinWidth = ng.Offsets.Left + ng.Offsets.Right;
            ng.MinHeight = ng.Offsets.Bottom + ng.Offsets.Top;
            ng.UpdateNines();
        }

        public void UpdateNines()
        {
            if (this.image.Any(i => i == null) || this.Source == null)
            {
                return;
            }

            double left = this.Offsets.Left / this.Source.Width;
            if (left > 1) left = 1;
            double right = this.Offsets.Right / this.Source.Width;
            if (right > 1) right = 1;
            double top = this.Offsets.Top / this.Source.Height;
            if (top > 1) top = 1;
            double bottom = this.Offsets.Bottom / this.Source.Height;
            if (bottom > 1) bottom = 1;

            double hmid = 1 - left - right;
            if (hmid < 0) hmid = 0;
            double vmid = 1 - bottom - top;
            if (vmid < 0) vmid = 0;

            this.image[0].Viewbox = new Rect(0, 0, left, top);
            this.image[1].Viewbox = new Rect(left, 0, hmid, top);
            this.image[2].Viewbox = new Rect(1 - right, 0, right, top);

            this.image[3].Viewbox = new Rect(0, top, left, vmid);
            this.image[4].Viewbox = new Rect(left, top, hmid, vmid);
            this.image[5].Viewbox = new Rect(1- right, top, right, vmid);

            this.image[6].Viewbox = new Rect(0, 1 - bottom, left, bottom);
            this.image[7].Viewbox = new Rect(left, 1 - bottom, hmid, bottom);
            this.image[8].Viewbox = new Rect(1 - right, 1 - bottom, right, bottom);
        }

        private void UpdateSizes()
        {
            if (this.topleft == null || this.bottomright == null || this.topmid == null || this.midleft == null)
                return;

            this.topleft.Width = this.Offsets.Left;
            this.topleft.Height = this.Offsets.Top;

            this.bottomright.Width = this.Offsets.Right;
            this.bottomright.Height = this.Offsets.Bottom;

            this.topmid.Width = Math.Max(this.ActualWidth - this.Offsets.Left - this.Offsets.Right, 0);

            this.midleft.Height = Math.Max(this.ActualHeight - this.Offsets.Bottom - this.Offsets.Top, 0);

            this.MinWidth = this.topleft.Width + this.bottomright.Width;
            this.MinHeight = this.topleft.Height + this.bottomright.Height;
        }
    }
}

I found a problem about the sizing.

In the UpdateSizes() method, these 2 lines are obsolete and prevent the automatic resize of the control when something changes

this.topmid.Width = Math.Max(this.ActualWidth - this.Offsets.Left - this.Offsets.Right, 0);
this.midleft.Height = Math.Max(this.ActualHeight - this.Offsets.Bottom - this.Offsets.Top, 0);

It fixed my issue, so maybe it could help someone else too.