fix: PictureBox.LoadAsync throws PlatformNotSupportedException

Async delegates have been deprecated in .NET Core (see https://github.com/dotnet/corefx/issues/5940)
for reasons such as:
* Async delegates use deprecated IAsyncResult-based async pattern.
This pattern is generally not supported throughout .NET Core base libraries.
* Async delegates depend on remoting (System.Runtime.Remoting) under the
hood. Remoting is not supported in .NET Core.

For more detailed reasons and migration strategies please refer to
https://devblogs.microsoft.com/dotnet/migrating-delegate-begininvoke-calls-for-net-core/

In case of WinForms, async pattern predates the Task-based Asynchronous
Pattern and it does not return awaitables.
In order to minimise the change to the public API surface we continue
to expose the existing API and just re-route the image loading routine
to a `Task` runner under the covers.

Fixes dotnet#242
Fixes dotnet#1548

src/System.Windows.Forms/src/System/Windows/Forms/PictureBox.cs

@@ -10,6 +10,7 @@
 using System.Net;
 using System.Runtime.InteropServices;
 using System.Threading;
+using System.Threading.Tasks;
 using System.Windows.Forms.Layout;
 
 namespace System.Windows.Forms
@@ -48,6 +49,7 @@ public class PictureBox : Control, ISupportInitialize
 
         // Instance members for asynchronous behavior
         private AsyncOperation _currentAsyncLoadOperation;
+
         private string _imageLocation;
         private Image _initialImage;
         private Image errorImage;
@@ -555,19 +557,11 @@ public void LoadAsync()
 
             WebRequest req = WebRequest.Create(CalculateUri(_imageLocation));
 
-            // Invoke BeginGetResponse on a threadpool thread, as it has
-            // unpredictable latency, since, on first call, it may load in the
-            // configuration system (this is NCL
-            (new WaitCallback(BeginGetResponseDelegate)).BeginInvoke(req, null, null);
-        }
-
-        /// <summary>
-        ///  Solely for calling BeginGetResponse itself asynchronously.
-        /// </summary>
-        private void BeginGetResponseDelegate(object arg)
-        {
-            WebRequest req = (WebRequest)arg;
-            req.BeginGetResponse(new AsyncCallback(GetResponseCallback), req);
+            Task.Run(() =>
+            {
+                // Invoke BeginGetResponse on a threadpool thread, as it has unpredictable latency
+                req.BeginGetResponse(new AsyncCallback(GetResponseCallback), req);
+            });
         }
 
         private void PostCompleted(Exception error, bool cancelled)

src/System.Windows.Forms/tests/IntegrationTests/WinFormsControlsClassicTests/WinFormsControlsClassicTests.csproj

@@ -201,6 +201,14 @@
       <Link>Panels.Designer.cs</Link>
       <DependentUpon>Panels.cs</DependentUpon>
     </Compile>
+    <Compile Include="..\WinformsControlsTest\PictureBoxes.cs">
+      <Link>PictureBoxes.cs</Link>
+      <SubType>Form</SubType>
+    </Compile>
+    <Compile Include="..\WinformsControlsTest\PictureBoxes.Designer.cs">
+      <Link>PictureBoxes.Designer.cs</Link>
+      <DependentUpon>PictureBoxes.cs</DependentUpon>
+    </Compile>
     <Compile Include="..\WinformsControlsTest\PropertyGrid.cs">
       <Link>PropertyGrid.cs</Link>
       <SubType>Form</SubType>
@@ -304,6 +312,10 @@
       <Link>Panels.resx</Link>
       <DependentUpon>Panels.cs</DependentUpon>
     </EmbeddedResource>
+    <EmbeddedResource Include="..\WinformsControlsTest\PictureBoxes.resx">
+      <Link>PictureBoxes.resx</Link>
+      <DependentUpon>PictureBoxes.cs</DependentUpon>
+    </EmbeddedResource>
     <EmbeddedResource Include="..\WinformsControlsTest\PropertyGrid.resx">
       <Link>PropertyGrid.resx</Link>
       <DependentUpon>PropertyGrid.cs</DependentUpon>
@@ -364,6 +376,4 @@
     </EmbeddedResource>
   </ItemGroup>
   <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
-  <Target Name="Build" Condition="'$(OS)' != 'Windows_NT'">
-  </Target>
 </Project>

src/System.Windows.Forms/tests/IntegrationTests/WinformsControlsTest/MainForm.cs

@@ -112,5 +112,10 @@ private void RichTextBoxes_Click(object sender, EventArgs e)
         {
             new RichTextBoxes().Show();
         }
+
+        private void PictureBoxes_Click(object sender, EventArgs e)
+        {
+            new PictureBoxes().Show();
+        }
     }
 }

src/System.Windows.Forms/tests/IntegrationTests/WinformsControlsTest/MainForm.resx

@@ -1,5 +1,64 @@
 <?xml version="1.0" encoding="utf-8"?>
 <root>
+  <!-- 
+    Microsoft ResX Schema 
+    
+    Version 2.0
+    
+    The primary goals of this format is to allow a simple XML format 
+    that is mostly human readable. The generation and parsing of the 
+    various data types are done through the TypeConverter classes 
+    associated with the data types.
+    
+    Example:
+    
+    ... ado.net/XML headers & schema ...
+    <resheader name="resmimetype">text/microsoft-resx</resheader>
+    <resheader name="version">2.0</resheader>
+    <resheader name="reader">System.Resources.ResXResourceReader, System.Windows.Forms, ...</resheader>
+    <resheader name="writer">System.Resources.ResXResourceWriter, System.Windows.Forms, ...</resheader>
+    <data name="Name1"><value>this is my long string</value><comment>this is a comment</comment></data>
+    <data name="Color1" type="System.Drawing.Color, System.Drawing">Blue</data>
+    <data name="Bitmap1" mimetype="application/x-microsoft.net.object.binary.base64">
+        <value>[base64 mime encoded serialized .NET Framework object]</value>
+    </data>
+    <data name="Icon1" type="System.Drawing.Icon, System.Drawing" mimetype="application/x-microsoft.net.object.bytearray.base64">
+        <value>[base64 mime encoded string representing a byte array form of the .NET Framework object]</value>
+        <comment>This is a comment</comment>
+    </data>
+                
+    There are any number of "resheader" rows that contain simple 
+    name/value pairs.
+    
+    Each data row contains a name, and value. The row also contains a 
+    type or mimetype. Type corresponds to a .NET class that support 
+    text/value conversion through the TypeConverter architecture. 
+    Classes that don't support this are serialized and stored with the 
+    mimetype set.
+    
+    The mimetype is used for serialized objects, and tells the 
+    ResXResourceReader how to depersist the object. This is currently not 
+    extensible. For a given mimetype the value must be set accordingly:
+    
+    Note - application/x-microsoft.net.object.binary.base64 is the format 
+    that the ResXResourceWriter will generate, however the reader can 
+    read any of the formats listed below.
+    
+    mimetype: application/x-microsoft.net.object.binary.base64
+    value   : The object must be serialized with 
+            : System.Runtime.Serialization.Formatters.Binary.BinaryFormatter
+            : and then encoded with base64 encoding.
+    
+    mimetype: application/x-microsoft.net.object.soap.base64
+    value   : The object must be serialized with 
+            : System.Runtime.Serialization.Formatters.Soap.SoapFormatter
+            : and then encoded with base64 encoding.
+
+    mimetype: application/x-microsoft.net.object.bytearray.base64
+    value   : The object must be serialized into a byte array 
+            : using a System.ComponentModel.TypeConverter
+            : and then encoded with base64 encoding.
+    -->
   <xsd:schema id="root" xmlns="" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
     <xsd:import namespace="http://www.w3.org/XML/1998/namespace" />
     <xsd:element name="root" msdata:IsDataSet="true">
@@ -58,4 +117,4 @@
   <resheader name="writer">
     <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </resheader>
-</root>
+</root>

src/System.Windows.Forms/tests/IntegrationTests/WinformsControlsTest/PictureBoxes.cs

@@ -0,0 +1,74 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Data;
+using System.Drawing;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows.Forms;
+
+namespace WinformsControlsTest
+{
+    public partial class PictureBoxes : Form
+    {
+        private const string Heading = "PictureBox tests";
+        private bool _isLoading;
+
+        public PictureBoxes()
+        {
+            InitializeComponent();
+            Text = Heading;
+        }
+
+        private void btnloadImage_Click(object sender, EventArgs e)
+        {
+            if (string.IsNullOrWhiteSpace(imageUri.Text))
+            {
+                pictureBox1.Image = null;
+                if (_isLoading)
+                {
+                    pictureBox1.CancelAsync();
+                }
+
+                return;
+            }
+
+            try
+            {
+                _isLoading = true;
+                pictureBox1.WaitOnLoad = false;
+                pictureBox1.LoadAsync(imageUri.Text);
+            }
+            catch (Exception ex)
+            {
+                MessageBox.Show(ex.Message);
+            }
+        }
+
+        private void pictureBox1_LoadCompleted(object sender, AsyncCompletedEventArgs e)
+        {
+            _isLoading = false;
+            Text = Heading;
+
+            if (e.Cancelled)
+            {
+                MessageBox.Show("Image loading cancelled");
+            }
+
+            if (e.Error != null)
+            {
+                MessageBox.Show(e.Error.Message, $"{e.Error.GetType().FullName} occurred");
+            }
+        }
+
+        private void pictureBox1_LoadProgressChanged(object sender, ProgressChangedEventArgs e)
+        {
+            Text = $"{Heading}: loading image progress: {e.ProgressPercentage}%";
+        }
+    }
+}