Add a lock to ProxyTypeEmitter (#90)

Attempt resolving race conditions as seen in aspnet/Hosting1312

src/Microsoft.Extensions.DiagnosticAdapter/Internal/ProxyTypeEmitter.cs

@@ -14,6 +14,7 @@ namespace Microsoft.Extensions.DiagnosticAdapter.Internal
     public static class ProxyTypeEmitter
     {
         private static readonly Type[] EmptyTypes = new Type[0];
+        private static object _lock = new object();
 
         public static Type GetProxyType(ProxyTypeCache cache, Type targetType, Type sourceType)
         {
@@ -164,8 +165,11 @@ namespace Microsoft.Extensions.DiagnosticAdapter.Internal
                     }
                     else
                     {
-                        // We need to proxy each of the elements. Let's generate a type.
-                        GenerateProxyTypeForList(elementKey.Item1, elementKey.Item2, proxyType, verificationResult);
+                        lock (_lock)
+                        {
+                            // We need to proxy each of the elements. Let's generate a type.
+                            GenerateProxyTypeForList(elementKey.Item1, elementKey.Item2, proxyType, verificationResult);
+                        }
                     }
 
                     return true;
@@ -241,7 +245,10 @@ namespace Microsoft.Extensions.DiagnosticAdapter.Internal
             }
 
             verificationResult.Mappings = propertyMappings;
-            GenerateProxyTypeFromProperties(sourceType, targetType, verificationResult);
+            lock (_lock)
+            {
+                GenerateProxyTypeFromProperties(sourceType, targetType, verificationResult);
+            }
 
             return true;
         }

test/Microsoft.Extensions.DiagnosticAdapter.Test/Internal/ProxyTypeEmitterTest.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading.Tasks;
 using Xunit;
 
 namespace Microsoft.Extensions.DiagnosticAdapter.Internal
@@ -623,6 +624,22 @@ namespace Microsoft.Extensions.DiagnosticAdapter.Internal
             Assert.Equal("Billy", proxy[0][0][0].FirstName);
         }
 
+        [Fact]
+        public void GetProxyType_LocksPreventDuplicateAssemblyNamesArgumentException_ForConcurrentThreads()
+        {
+            for (var i = 0; i < 5; i++)
+            {
+                Parallel.For(
+                0,
+                100,
+                (j) =>
+                {
+                    var testObject = new Person();
+                    ProxyTypeEmitter.GetProxyType(new ProxyTypeCache(), typeof(IPerson), testObject.GetType());
+                });
+            }
+        }
+
         private object ConvertTo(object value, Type type)
         {
             var cache = new ProxyTypeCache();