ImageFrameCollection now properly implements IDisposable interface & ensures all operations are called on valid object

src/ImageSharp/ImageFrameCollection.cs

@@ -11,8 +11,10 @@ namespace SixLabors.ImageSharp
     /// Encapsulates a pixel-agnostic collection of <see cref="ImageFrame"/> instances
     /// that make up an <see cref="Image"/>.
     /// </summary>
-    public abstract class ImageFrameCollection : IEnumerable<ImageFrame>
+    public abstract class ImageFrameCollection : IDisposable, IEnumerable<ImageFrame>
     {
+        private bool isDisposed;
+
         /// <summary>
         /// Gets the number of frames.
         /// </summary>
@@ -21,7 +23,15 @@ namespace SixLabors.ImageSharp
         /// <summary>
         /// Gets the root frame.
         /// </summary>
-        public ImageFrame RootFrame => this.NonGenericRootFrame;
+        public ImageFrame RootFrame
+        {
+            get
+            {
+                this.EnsureNotDisposed();
+
+                return this.NonGenericRootFrame;
+            }
+        }
 
         /// <summary>
         /// Gets the root frame. (Implements <see cref="RootFrame"/>.)
@@ -36,7 +46,15 @@ namespace SixLabors.ImageSharp
         /// </value>
         /// <param name="index">The index.</param>
         /// <returns>The <see cref="ImageFrame"/> at the specified index.</returns>
-        public ImageFrame this[int index] => this.NonGenericGetFrame(index);
+        public ImageFrame this[int index]
+        {
+            get
+            {
+                this.EnsureNotDisposed();
+
+                return this.NonGenericGetFrame(index);
+            }
+        }
 
         /// <summary>
         /// Determines the index of a specific <paramref name="frame"/> in the <seealso cref="ImageFrameCollection"/>.
@@ -59,7 +77,12 @@ namespace SixLabors.ImageSharp
         /// </summary>
         /// <param name="source">The raw pixel data to generate the <seealso cref="ImageFrame{TPixel}"/> from.</param>
         /// <returns>The cloned <see cref="ImageFrame{TPixel}"/>.</returns>
-        public ImageFrame AddFrame(ImageFrame source) => this.NonGenericAddFrame(source);
+        public ImageFrame AddFrame(ImageFrame source)
+        {
+            this.EnsureNotDisposed();
+
+            return this.NonGenericAddFrame(source);
+        }
 
         /// <summary>
         /// Removes the frame at the specified index and frees all freeable resources associated with it.
@@ -91,7 +114,12 @@ namespace SixLabors.ImageSharp
         /// <param name="index">The zero-based index of the frame to export.</param>
         /// <exception cref="InvalidOperationException">Cannot remove last frame.</exception>
         /// <returns>The new <see cref="Image{TPixel}"/> with the specified frame.</returns>
-        public Image ExportFrame(int index) => this.NonGenericExportFrame(index);
+        public Image ExportFrame(int index)
+        {
+            this.EnsureNotDisposed();
+
+            return this.NonGenericExportFrame(index);
+        }
 
         /// <summary>
         /// Creates an <see cref="Image{T}"/> with only the frame at the specified index
@@ -99,7 +127,12 @@ namespace SixLabors.ImageSharp
         /// </summary>
         /// <param name="index">The zero-based index of the frame to clone.</param>
         /// <returns>The new <see cref="Image{TPixel}"/> with the specified frame.</returns>
-        public Image CloneFrame(int index) => this.NonGenericCloneFrame(index);
+        public Image CloneFrame(int index)
+        {
+            this.EnsureNotDisposed();
+
+            return this.NonGenericCloneFrame(index);
+        }
 
         /// <summary>
         /// Creates a new <seealso cref="ImageFrame{TPixel}" /> and appends it to the end of the collection.
@@ -107,7 +140,12 @@ namespace SixLabors.ImageSharp
         /// <returns>
         /// The new <see cref="ImageFrame{TPixel}" />.
         /// </returns>
-        public ImageFrame CreateFrame() => this.NonGenericCreateFrame();
+        public ImageFrame CreateFrame()
+        {
+            this.EnsureNotDisposed();
+
+            return this.NonGenericCreateFrame();
+        }
 
         /// <summary>
         /// Creates a new <seealso cref="ImageFrame{TPixel}" /> and appends it to the end of the collection.
@@ -116,14 +154,53 @@ namespace SixLabors.ImageSharp
         /// <returns>
         /// The new <see cref="ImageFrame{TPixel}" />.
         /// </returns>
-        public ImageFrame CreateFrame(Color backgroundColor) => this.NonGenericCreateFrame(backgroundColor);
+        public ImageFrame CreateFrame(Color backgroundColor)
+        {
+            this.EnsureNotDisposed();
+
+            return this.NonGenericCreateFrame(backgroundColor);
+        }
 
         /// <inheritdoc />
-        public IEnumerator<ImageFrame> GetEnumerator() => this.NonGenericGetEnumerator();
+        public void Dispose()
+        {
+            if (this.isDisposed)
+            {
+                return;
+            }
+
+            this.DisposeManaged();
+
+            this.isDisposed = true;
+        }
+
+        /// <inheritdoc />
+        public IEnumerator<ImageFrame> GetEnumerator()
+        {
+            this.EnsureNotDisposed();
+
+            return this.NonGenericGetEnumerator();
+        }
 
         /// <inheritdoc/>
         IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();
 
+        /// <summary>
+        /// Throws <see cref="ObjectDisposedException"/> if the image frame is disposed.
+        /// </summary>
+        protected void EnsureNotDisposed()
+        {
+            if(this.isDisposed)
+            {
+                throw new ObjectDisposedException("Trying to execute an operation on a disposed image frame.");
+            }
+        }
+
+        /// <summary>
+        /// Internal routine for freeing managed resources called from <see cref="Dispose"/>
+        /// </summary>
+        protected abstract void DisposeManaged();
+
         /// <summary>
         /// Implements <see cref="GetEnumerator"/>.
         /// </summary>

src/ImageSharp/ImageFrameCollection{TPixel}.cs

@@ -335,7 +335,8 @@ namespace SixLabors.ImageSharp
             }
         }
 
-        internal void Dispose()
+        /// <inheritdoc/>
+        protected override void DisposeManaged()
         {
             foreach (ImageFrame<TPixel> f in this.frames)
             {