Added the image indexer, used to distinguish between image indexes an…

…d image keys.

src/BetterControls/Drawing/ImageIndexer.cs

@@ -0,0 +1,77 @@
+using System.Windows.Forms;
+
+namespace BetterControls.Drawing
+{
+    /// <summary>
+    /// Used to provide a mechanism for a control to support using both image indexes and image keys.
+    /// </summary>
+    public class ImageIndexer
+    {
+        private ImageList _imageList;
+        private int _index = -1;
+        private string _key = string.Empty;
+        private bool _useIndex = false;
+
+        /// <summary>
+        /// Gets the image list associated with this image indexer.
+        /// </summary>
+        protected virtual ImageList ImageList => _imageList;
+
+        /// <summary>
+        /// Gets or sets the index of the image from the associated image list. The image indexer is configured to use this index as an image source.
+        /// </summary>
+        public virtual int Index
+        {
+            get => _index;
+            set
+            {
+                if (Index != value)
+                {
+                    _index = value;
+                }
+
+                _useIndex = true;
+            }
+        }
+
+        /// <summary>
+        /// Gets or sets the key of the image from the associated image list. The image indexer is configured to use this key as an image source.
+        /// </summary>
+        public virtual string Key
+        {
+            get => _key;
+            set
+            {
+                if (Key != value)
+                {
+                    if (value is null)
+                        _key = string.Empty;
+                    else
+                        _key = value;
+                }
+
+                _useIndex = false;
+            }
+        }
+
+        /// <summary>
+        /// Gets the computed image index, based on whether the image indexer is configured to use the index or key as an image source.
+        /// </summary>
+        public virtual int ComputedIndex
+        {
+            get
+            {
+                if (_useIndex)
+                {
+                    return Index;
+                }
+                else if (ImageList != null)
+                {
+                    return ImageList.Images.IndexOfKey(Key);
+                }
+
+                return -1;
+            }
+        }
+    }
+}