[FileExplorer] StlThumbnailProvider (#15568)

* Adds StlThumbnailProvider

* Spell checker fixes

* Adds missing changes

* Attempts to fix alpha background issue

* Adds missing dependency references

* Upgrades .NET Core 3.1 to .NET 5

* Updates Helix Toolkit to fix .net5 compatibility

* Return null bitmap If STL model is empty

src/modules/previewpane/UnitTests-StlThumbnailProvider/StlThumbnailProviderTests.cs

@@ -0,0 +1,111 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Drawing;
+using System.IO;
+using System.Runtime.InteropServices;
+using System.Runtime.InteropServices.ComTypes;
+using Common.ComInterlop;
+using Microsoft.PowerToys.STATestExtension;
+using Microsoft.PowerToys.ThumbnailHandler.Stl;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Moq;
+
+namespace StlThumbnailProviderUnitTests
+{
+    [STATestClass]
+    public class StlThumbnailProviderTests
+    {
+        [TestMethod]
+        public void GetThumbnailValidStreamStl()
+        {
+            // Act
+            var file = File.ReadAllBytes("HelperFiles/sample.stl");
+
+            StlThumbnailProvider provider = new StlThumbnailProvider();
+
+            provider.Initialize(GetMockStream(file), 0);
+
+            provider.GetThumbnail(256, out IntPtr bitmap, out WTS_ALPHATYPE alphaType);
+
+            Assert.IsTrue(bitmap != IntPtr.Zero);
+            Assert.IsTrue(alphaType == WTS_ALPHATYPE.WTSAT_ARGB);
+        }
+
+        [TestMethod]
+        public void GetThumbnailInValidSizeStl()
+        {
+            // Act
+            var file = File.ReadAllBytes("HelperFiles/sample.stl");
+
+            StlThumbnailProvider provider = new StlThumbnailProvider();
+
+            provider.Initialize(GetMockStream(file), 0);
+
+            provider.GetThumbnail(0, out IntPtr bitmap, out WTS_ALPHATYPE alphaType);
+
+            Assert.IsTrue(bitmap == IntPtr.Zero);
+            Assert.IsTrue(alphaType == WTS_ALPHATYPE.WTSAT_UNKNOWN);
+        }
+
+        [TestMethod]
+        public void GetThumbnailToBigStl()
+        {
+            // Act
+            var file = File.ReadAllBytes("HelperFiles/sample.stl");
+
+            StlThumbnailProvider provider = new StlThumbnailProvider();
+
+            provider.Initialize(GetMockStream(file), 0);
+
+            provider.GetThumbnail(10001, out IntPtr bitmap, out WTS_ALPHATYPE alphaType);
+
+            Assert.IsTrue(bitmap == IntPtr.Zero);
+            Assert.IsTrue(alphaType == WTS_ALPHATYPE.WTSAT_UNKNOWN);
+        }
+
+        [TestMethod]
+        public void CheckNoStlEmptyStreamShouldReturnNullBitmap()
+        {
+            using (var stream = new MemoryStream())
+            {
+                Bitmap thumbnail = StlThumbnailProvider.GetThumbnail(stream, 256);
+                Assert.IsTrue(thumbnail == null);
+            }
+        }
+
+        [TestMethod]
+        public void CheckNoStlNullStreamShouldReturnNullBitmap()
+        {
+            Bitmap thumbnail = StlThumbnailProvider.GetThumbnail(null, 256);
+            Assert.IsTrue(thumbnail == null);
+        }
+
+        private static IStream GetMockStream(byte[] sourceArray)
+        {
+            var streamMock = new Mock<IStream>();
+            int bytesRead = 0;
+
+            streamMock
+                .Setup(x => x.Read(It.IsAny<byte[]>(), It.IsAny<int>(), It.IsAny<IntPtr>()))
+                .Callback<byte[], int, IntPtr>((buffer, countToRead, bytesReadPtr) =>
+                {
+                    int actualCountToRead = Math.Min(sourceArray.Length - bytesRead, countToRead);
+                    if (actualCountToRead > 0)
+                    {
+                        Array.Copy(sourceArray, bytesRead, buffer, 0, actualCountToRead);
+                        Marshal.WriteInt32(bytesReadPtr, actualCountToRead);
+                        bytesRead += actualCountToRead;
+                    }
+                    else
+                    {
+                        Marshal.WriteInt32(bytesReadPtr, 0);
+                    }
+                });
+
+            return streamMock.Object;
+        }
+    }
+}