Replace PartialList with ReadOnlyCollection (#9890)

* Replace PartialList with ReadOnlyCollection

* PR feedback

Author: ThomasGoulet73

src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Imaging/BitmapPalette.cs

@@ -4,6 +4,7 @@
 
 using MS.Internal;
 using MS.Win32.PresentationCore;
+using System.Collections.ObjectModel;
 using System.Windows.Threading;
 using System.Runtime.InteropServices;
 
@@ -44,7 +45,7 @@ public BitmapPalette(IList<Color> colors)
                 colorArray[i] = colors[i];
             }
 
-            _colors = new PartialList<Color>(colorArray);
+            _colors = new ReadOnlyCollection<Color>(colorArray);
 
             _palette = CreateInternalPalette();
 
@@ -330,7 +331,7 @@ private void UpdateManaged()
                 }
             }
 
-            _colors = new PartialList<Color>(colors);
+            _colors = new ReadOnlyCollection<Color>(colors);
         }
 
         #endregion // Private Methods
@@ -365,7 +366,7 @@ private struct ImagePaletteColor
         // the behavior that we want.
         private SafeMILHandle _palette = null; // IWICPalette*
 
-        private IList<Color> _colors = new PartialList<Color>(new List<Color>());
+        private IList<Color> _colors = ReadOnlyCollection<Color>.Empty;
     }
 }
 

src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/PixelFormat.cs

@@ -63,7 +63,7 @@ public IList<byte> Mask
         {
             get
             {
-                return _mask != null ? new PartialList<byte>((byte[])_mask.Clone()) : null;
+                return _mask != null ? new ReadOnlyCollection<byte>((byte[])_mask.Clone()) : null;
             }
         }
 

src/Microsoft.DotNet.Wpf/src/Shared/MS/Internal/PartialList.cs

@@ -30,17 +30,6 @@ internal class PartialList<T> : IList<T>
         private int _initialIndex;
         private int _count;
 
-        /// <summary>
-        /// Convenience constructor for taking in an entire list. Useful for creating a read-only
-        /// version of the list.
-        /// </summary>
-        public PartialList(IList<T> list)
-        {
-            _list = list;
-            _initialIndex = 0;
-            _count = list.Count;
-        }
-
         public PartialList(IList<T> list, int initialIndex, int count)
         {
             // make sure early that the caller didn't miscalculate index and count
@@ -51,22 +40,6 @@ public PartialList(IList<T> list, int initialIndex, int count)
             _count = count;
         }
 
-#if !PRESENTATION_CORE
-        /// <summary>
-        /// Creates new PartialList object only for true partial ranges.
-        /// Otherwise, returns the original list.
-        /// </summary>
-        public static IList<T> Create(IList<T> list, int initialIndex, int count)
-        {
-            if (list == null)
-                return null;
-
-            if (initialIndex == 0 && count == list.Count)
-                return list;
-
-            return new PartialList<T>(list, initialIndex, count);
-        }
-#endif
         #region IList<T> Members
 
         public void RemoveAt(int index)