Improved dispose pattern in ChromiumWebBrowser (#2651)

* Code Cleanup - Improved ChromiumWebBrowser Dispose/Finalise

* Added thread safe dispose pattern
* Added IsDisposed

* Wpf - Cleanup and improvements to the dispose pattern

* WinForms - Cleanup and improvements to the dispose pattern

* OffScreen - Cleanup and improvements to the dispose pattern

* Added IsDisposed to IWebBrowser

* Fixed comments

* WinForms - Removed Finalizer and Dispose()

* WPF - ChromiumWebBrowser.Dispose add comment for Interlocked.CompareExchange

* WinForms - ChromiumWebBrowser.Dispose add comment for Interlocked.CompareExchange

Author: merceyz

CefSharp.WinForms/ChromiumWebBrowser.cs

@@ -6,6 +6,7 @@
 using System.ComponentModel;
 using System.Drawing;
 using System.Runtime.CompilerServices;
+using System.Threading;
 using System.Windows.Forms;
 using CefSharp.Internals;
 using CefSharp.WinForms.Internals;
@@ -59,6 +60,25 @@ public class ChromiumWebBrowser : Control, IWebBrowserInternal, IWinFormsWebBrow
         /// </summary>
         private IRequestContext requestContext;
 
+        /// <summary>
+        /// The value for disposal, if it's 1 (one) then this instance is either disposed
+        /// or in the process of getting disposed
+        /// </summary>
+        private int disposeSignaled;
+
+        /// <summary>
+        /// Gets a value indicating whether this instance is disposed.
+        /// </summary>
+        /// <value><see langword="true" /> if this instance is disposed; otherwise, <see langword="false" />.</value>
+        [Browsable(false), EditorBrowsable(EditorBrowsableState.Never), DefaultValue(false)]
+        public new bool IsDisposed
+        {
+            get
+            {
+                return Interlocked.CompareExchange(ref disposeSignaled, 1, 1) == 1;
+            }
+        }
+
         /// <summary>
         /// Set to true while handing an activating WM_ACTIVATE message.
         /// MUST ONLY be cleared by DefaultFocusHandler.
@@ -431,73 +451,71 @@ private void InitializeFieldsAndCefIfRequired()
         }
 
         /// <summary>
-        /// Releases the unmanaged resources used by the <see cref="T:System.Windows.Forms.Control" /> and its child controls and optionally releases the managed resources.
+        /// If not in design mode; Releases unmanaged and - optionally - managed resources for the <see cref="ChromiumWebBrowser"/>
         /// </summary>
-        /// <param name="disposing">true to release both managed and unmanaged resources; false to release only unmanaged resources.</param>
+        /// <param name="disposing"><see langword="true" /> to release both managed and unmanaged resources; <see langword="false" /> to release only unmanaged resources.</param>
         protected override void Dispose(bool disposing)
         {
-            IsBrowserInitialized = false;
-
-            if (!designMode)
+            // Attempt to move the disposeSignaled state from 0 to 1. If successful, we can be assured that
+            // this thread is the first thread to do so, and can safely dispose of the object.
+            if (Interlocked.CompareExchange(ref disposeSignaled, 1, 0) != 0)
             {
-                RemoveFromListOfCefBrowsers();
+                return;
             }
 
-            //The unmanaged resources should never be created in design mode, so only dispose when
-            //at runtime
-            if (disposing && !designMode)
+            if (!designMode)
             {
-                FreeUnmanagedResources();
+                InternalDispose(disposing);
             }
 
-            // Don't maintain a reference to event listeners anylonger:
-            AddressChanged = null;
-            ConsoleMessage = null;
-            FrameLoadEnd = null;
-            FrameLoadStart = null;
-            IsBrowserInitializedChanged = null;
-            LoadError = null;
-            LoadingStateChanged = null;
-            StatusMessage = null;
-            TitleChanged = null;
-
-            // Release reference to handlers, make sure this is done after we dispose managedCefBrowserAdapter
-            // otherwise the ILifeSpanHandler.DoClose will not be invoked.
-            this.SetHandlersToNull();
-
             base.Dispose(disposing);
         }
 
         /// <summary>
-        /// Required for designer support - this method cannot be inlined as the designer
-        /// will attempt to load libcef.dll and will subsiquently throw an exception.
+        /// Releases unmanaged and - optionally - managed resources for the <see cref="ChromiumWebBrowser"/>
         /// </summary>
+        /// <param name="disposing"><see langword="true" /> to release both managed and unmanaged resources; <see langword="false" /> to release only unmanaged resources.</param>
+        /// <remarks>
+        /// This method cannot be inlined as the designer will attempt to load libcef.dll and will subsiquently throw an exception.
+        /// </remarks>
         [MethodImpl(MethodImplOptions.NoInlining)]
-        private void RemoveFromListOfCefBrowsers()
+        private void InternalDispose(bool disposing)
         {
-            Cef.RemoveDisposable(this);
-        }
+            if (disposing)
+            {
+                IsBrowserInitialized = false;
 
-        /// <summary>
-        /// Required for designer support - this method cannot be inlined as the designer
-        /// will attempt to load libcef.dll and will subsiquently throw an exception.
-        /// </summary>
-        [MethodImpl(MethodImplOptions.NoInlining)]
-        private void FreeUnmanagedResources()
-        {
-            browser = null;
+                browser = null;
 
-            if (parentFormMessageInterceptor != null)
-            {
-                parentFormMessageInterceptor.Dispose();
-                parentFormMessageInterceptor = null;
-            }
+                if (parentFormMessageInterceptor != null)
+                {
+                    parentFormMessageInterceptor.Dispose();
+                    parentFormMessageInterceptor = null;
+                }
+                
+                if (managedCefBrowserAdapter != null)
+                {
+                    managedCefBrowserAdapter.Dispose();
+                    managedCefBrowserAdapter = null;
+                }
 
-            if (managedCefBrowserAdapter != null)
-            {
-                managedCefBrowserAdapter.Dispose();
-                managedCefBrowserAdapter = null;
+                // Don't maintain a reference to event listeners anylonger:
+                AddressChanged = null;
+                ConsoleMessage = null;
+                FrameLoadEnd = null;
+                FrameLoadStart = null;
+                IsBrowserInitializedChanged = null;
+                LoadError = null;
+                LoadingStateChanged = null;
+                StatusMessage = null;
+                TitleChanged = null;
+
+                // Release reference to handlers, make sure this is done after we dispose managedCefBrowserAdapter
+                // otherwise the ILifeSpanHandler.DoClose will not be invoked.
+                this.SetHandlersToNull();
             }
+
+            Cef.RemoveDisposable(this);
         }
 
         /// <summary>