CSHARP-2595: Refactor BsonClassMap to work around changes in .NET Core 3.0.

Author: rstam

src/MongoDB.Bson/MongoDB.Bson.csproj

@@ -50,6 +50,7 @@
     <PackageReference Include="System.Diagnostics.Process" Version="4.1.0" />
     <PackageReference Include="System.Dynamic.Runtime" Version="4.0.11" />
     <PackageReference Include="System.Reflection.Emit.Lightweight" Version="4.3.0" />
+    <PackageReference Include="System.Runtime.Serialization.Formatters" Version="4.3.0" />
   </ItemGroup>
 
   <ItemGroup>

src/MongoDB.Bson/Serialization/BsonClassMap.cs

@@ -20,9 +20,7 @@
 using System.Linq.Expressions;
 using System.Reflection;
 using System.Runtime.CompilerServices;
-#if NET452
 using System.Runtime.Serialization;
-#endif
 using MongoDB.Bson.IO;
 using MongoDB.Bson.Serialization.Conventions;
 
@@ -36,13 +34,8 @@ public class BsonClassMap
         // private static fields
         private readonly static Dictionary<Type, BsonClassMap> __classMaps = new Dictionary<Type, BsonClassMap>();
         private readonly static Queue<Type> __knownTypesQueue = new Queue<Type>();
-
-        private static readonly MethodInfo __getUninitializedObjectMethodInfo =
-            GetFormatterServicesType()
-            .GetTypeInfo()
-            ?.GetMethod("GetUninitializedObject", BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static);
-
         private static int __freezeNestingLevel = 0;
+        private static readonly MethodInfo __getUninitializedObjectMethodInfo = GetGetUninitializedObjectMethodInfo();
 
         // private fields
         private readonly Type _classType;
@@ -423,6 +416,55 @@ public static void RegisterClassMap(BsonClassMap classMap)
             }
         }
 
+        // private static methods
+        private static Type GetFormatterServicesType()
+        {
+#if NET452
+            return typeof(FormatterServices);
+#else
+            // TODO: once we depend on newer versions of .NET Standard we should be able to do this without reflection
+
+            try
+            {
+                // new approach which works on .NET Core 3.0
+                var formattersAssembly = Assembly.Load(new AssemblyName("System.Runtime.Serialization.Formatters"));
+                var formatterServicesType = formattersAssembly.GetType("System.Runtime.Serialization.FormatterServices");
+                if (formatterServicesType != null)
+                {
+                    return formatterServicesType;
+                }
+            }
+            catch
+            {
+                // ignore exceptions and continue to fallback code
+            }
+
+            // fallback to previous approach (which worked on older versions of .NET Core)
+            var mscorlibAssembly = typeof(string).GetTypeInfo().Assembly;
+            return mscorlibAssembly.GetType("System.Runtime.Serialization.FormatterServices");
+#endif
+        }
+
+        private static MethodInfo GetGetUninitializedObjectMethodInfo()
+        {
+            // don't let exceptions leak out of this method because it's called from the type initializer
+            try
+            {
+                var formatterServicesType = GetFormatterServicesType();
+                if (formatterServicesType != null)
+                {
+                    var formatterServicesTypeInfo = formatterServicesType.GetTypeInfo();
+                    return formatterServicesTypeInfo.GetMethod("GetUninitializedObject", BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static);
+                }
+            }
+            catch
+            {
+                // ignore exceptions
+            }
+
+            return null;
+        }
+
         // public methods
         /// <summary>
         /// Automaps the class.
@@ -1303,21 +1345,7 @@ private void ThrowNotFrozenException()
             var message = string.Format("Class map for {0} has been not been frozen yet.", _classType.FullName);
             throw new InvalidOperationException(message);
         }
-
-        private static Type GetFormatterServicesType()
-        {
-#if NET452
-            return typeof(string)
-                .GetTypeInfo()
-                .Assembly
-                .GetType("System.Runtime.Serialization.FormatterServices");
-#else
-            return 
-                Assembly.Load(new AssemblyName("System.Runtime.Serialization.Formatters"))
-                .GetType("System.Runtime.Serialization.FormatterServices");
-#endif
-        }
-	}
+    }
 
     /// <summary>
     /// Represents a mapping between a class and a BSON document.