Fix X509CertificateCollection.SyncRoot

Not only is it allocating on every call, but it's returning null on the
first call.

Author: stephentoub

src/System.Security.Cryptography.X509Certificates/src/System/Security/Cryptography/X509Certificates/X509CertificateCollection.cs

@@ -9,6 +9,7 @@
 using System.Globalization;
 using System.Collections.Generic;
 using System.Runtime.InteropServices;
+using System.Threading;
 
 using Interlocked = System.Threading.Interlocked;
 
@@ -49,7 +50,11 @@ Object ICollection.SyncRoot
         {
             get
             {
-                return Interlocked.CompareExchange(ref _syncRoot, new Object(), null);
+                if (_syncRoot == null)
+                {
+                    Interlocked.CompareExchange(ref _syncRoot, new object(), null);
+                }
+                return _syncRoot;
             }
         }
 

src/System.Security.Cryptography.X509Certificates/tests/CollectionTests.cs

@@ -317,6 +317,14 @@ public static void ExportPkcs7()
             TestExportStore(X509ContentType.Pkcs7);
         }
 
+        [Fact]
+        public static void X509CertificateCollectionSyncRoot()
+        {
+            var cc = new X509CertificateCollection();
+            Assert.NotNull(((ICollection)cc).SyncRoot);
+            Assert.Same(((ICollection)cc).SyncRoot, ((ICollection)cc).SyncRoot);
+        }
+
         private static void TestExportSingleCert(X509ContentType ct)
         {
             using (var msCer = new X509Certificate2(TestData.MsCertificate))