Fix exceptions in .NET Standard projects other than .NET Core

The System.Runtime.Loader is supported only for .NET Core despite
having .NET Standard target, and was causing
TypeInitializationExceptions. All usage of this class was moved to
separate util class so this exception can be caught properly.
Also it seems that DependencyContext.Default is returning null for
other than .NET Core projects so null checks were added as well.

DEVSIX-1726

Author: IdamkinI

itext/itext.io/itext/io/util/AssemblyLoadContextUtil.cs

@@ -0,0 +1,84 @@
+/*
+
+This file is part of the iText (R) project.
+Copyright (c) 1998-2018 iText Group NV
+Authors: Bruno Lowagie, Paulo Soares, et al.
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License version 3
+as published by the Free Software Foundation with the addition of the
+following permission added to Section 15 as permitted in Section 7(a):
+FOR ANY PART OF THE COVERED WORK IN WHICH THE COPYRIGHT IS OWNED BY
+ITEXT GROUP. ITEXT GROUP DISCLAIMS THE WARRANTY OF NON INFRINGEMENT
+OF THIRD PARTY RIGHTS
+
+This program is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+or FITNESS FOR A PARTICULAR PURPOSE.
+See the GNU Affero General Public License for more details.
+You should have received a copy of the GNU Affero General Public License
+along with this program; if not, see http://www.gnu.org/licenses or write to
+the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+Boston, MA, 02110-1301 USA, or download the license from the following URL:
+http://itextpdf.com/terms-of-use/
+
+The interactive user interfaces in modified source and object code versions
+of this program must display Appropriate Legal Notices, as required under
+Section 5 of the GNU Affero General Public License.
+
+In accordance with Section 7(b) of the GNU Affero General Public License,
+a covered work must retain the producer line in every PDF that is created
+or manipulated using iText.
+
+You can be released from the requirements of the license by purchasing
+a commercial license. Buying such a license is mandatory as soon as you
+develop commercial activities involving the iText software without
+disclosing the source code of your own applications.
+These activities include: offering paid services to customers as an ASP,
+serving PDFs on the fly in a web application, shipping iText with a closed
+source product.
+
+For more information, please contact iText Software Corp. at this
+address: [email protected]
+*/
+using System;
+using System.Collections.Generic;
+using System.Reflection;
+using System.Text;
+
+namespace iText.IO.Util
+{
+#if NETSTANDARD1_6
+    /// <summary>This file is a helper class for internal usage only.</summary>
+    /// <remarks>
+    /// This file is a helper class for internal usage only.
+    /// Be aware that it's API and functionality may be changed in future.
+    /// <br/>
+    /// <br/>
+    /// Ussage of this class may throw TypeInitializationException
+    /// in .NET Standard enviroments that doesn't support System.Runtime.Loader
+    /// </remarks>
+    public static class AssemblyLoadContextUtil {
+
+        public static AssemblyName GetAssemblyName(String path) {
+            return System.Runtime.Loader.AssemblyLoadContext.GetAssemblyName(path);
+        }
+
+        public static Assembly LoadFromDefaultContextAssemblyPath(String path) {
+            return System.Runtime.Loader.AssemblyLoadContext.Default.LoadFromAssemblyPath(path);
+        }
+
+        public static void RegisterUnloadingEvent(Action<object> onUnloading) {
+            RegisterUnloadingEvent(onUnloading, null);
+        }
+
+        public static void RegisterUnloadingEvent(Action<object> onUnloading, Assembly assembly) {
+            if (assembly == null) {
+                System.Runtime.Loader.AssemblyLoadContext.Default.Unloading += onUnloading;
+            } else {
+                System.Runtime.Loader.AssemblyLoadContext.GetLoadContext(assembly).Unloading += onUnloading;
+            }
+        }
+    }
+#endif
+}

itext/itext.io/itext/io/util/ResourceUtil.cs

@@ -48,7 +48,6 @@ source product.
 using System.Linq;
 using System.Reflection;
 #if NETSTANDARD1_6
-using System.Runtime.Loader;
 using Microsoft.DotNet.PlatformAbstractions;
 using Microsoft.Extensions.DependencyModel;
 #endif
@@ -134,17 +133,19 @@ public static Stream GetResourceStream(string key, Type definedClassType) {
                 }
             }
 #else
-            string runtimeId = RuntimeEnvironment.GetRuntimeIdentifier();
-            IEnumerable<AssemblyName> loadedAssemblies = DependencyContext.Default.GetRuntimeAssemblyNames(runtimeId).ToList();
-            foreach (AssemblyName assemblyName in loadedAssemblies) {
-                if (assemblyName.Name.StartsWith("itext")) {
-                    try {
-                        Assembly assembly = Assembly.Load(assemblyName);
-                        istr = SearchResourceInAssembly(key, assembly);
-                        if (istr != null) {
-                            return istr;
-                        }
-                    } catch { }
+            if (DependencyContext.Default != null) {
+                string runtimeId = RuntimeEnvironment.GetRuntimeIdentifier();
+                IEnumerable<AssemblyName> loadedAssemblies = DependencyContext.Default.GetRuntimeAssemblyNames(runtimeId).ToList();
+                foreach (AssemblyName assemblyName in loadedAssemblies) {
+                    if (assemblyName.Name.StartsWith("itext")) {
+                        try {
+                            Assembly assembly = Assembly.Load(assemblyName);
+                            istr = SearchResourceInAssembly(key, assembly);
+                            if (istr != null) {
+                                return istr;
+                            }
+                        } catch { }
+                    }
                 }
             }
 #endif
@@ -170,7 +171,7 @@ private static Stream SearchResourceInAssembly(string key, Object obj) {
 #if !NETSTANDARD1_6
                         istr = Assembly.LoadFrom(dir).GetManifestResourceStream(key);
 #else
-                        istr = AssemblyLoadContext.Default.LoadFromAssemblyPath(key).GetManifestResourceStream(key);
+                        istr = AssemblyLoadContextUtil.LoadFromDefaultContextAssemblyPath(key).GetManifestResourceStream(key);
 #endif
                     }
                     catch
@@ -234,16 +235,16 @@ private static void LoadITextResourceAssemblies() {
             }
 #else
             string runtimeId = RuntimeEnvironment.GetRuntimeIdentifier();
-            List<AssemblyName> loadedAssemblies = DependencyContext.Default.GetRuntimeAssemblyNames(runtimeId).ToList();
+            List<AssemblyName> loadedAssemblies = DependencyContext.Default != null ? DependencyContext.Default.GetRuntimeAssemblyNames(runtimeId).ToList() : new List<AssemblyName>();
 
             var referencedPaths = Directory.GetFiles(FileUtil.GetBaseDirectory(), "*.dll");
             foreach (String path in referencedPaths)
             {
                 try
                 {
-                    AssemblyName name = AssemblyLoadContext.GetAssemblyName(path);
+                    AssemblyName name = AssemblyLoadContextUtil.GetAssemblyName(path);
                     if (iTextResourceAssemblyNames.Contains(name.Name) && !loadedAssemblies.Any(assembly => assembly.Name.Equals(name.Name))) {
-                        Assembly newAssembly = AssemblyLoadContext.Default.LoadFromAssemblyPath(path);
+                        Assembly newAssembly = AssemblyLoadContextUtil.LoadFromDefaultContextAssemblyPath(path);
                         loadedAssemblies.Add(newAssembly.GetName());
                     }
                 }