Make reading key usages from X509EnhancedKeyUsageExtension thread safe

src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/X509EnhancedKeyUsageExtension.cs

@@ -30,7 +30,7 @@ public OidCollection EnhancedKeyUsages
             {
                 if (!_decoded)
                 {
-                    DecodeX509EnhancedKeyUsageExtension(RawData, out _enhancedKeyUsages);
+                    _enhancedKeyUsages = DecodeX509EnhancedKeyUsageExtension(RawData);
                     _decoded = true;
                 }
 
@@ -77,7 +77,7 @@ private static byte[] EncodeExtension(OidCollection enhancedKeyUsages)
             return writer.Encode();
         }
 
-        private static void DecodeX509EnhancedKeyUsageExtension(byte[] encoded, out OidCollection usages)
+        private static OidCollection DecodeX509EnhancedKeyUsageExtension(byte[] encoded)
         {
             // https://tools.ietf.org/html/rfc5924#section-4.1
             //
@@ -90,12 +90,14 @@ private static void DecodeX509EnhancedKeyUsageExtension(byte[] encoded, out OidC
                 ValueAsnReader reader = new(encoded, AsnEncodingRules.BER);
                 ValueAsnReader sequenceReader = reader.ReadSequence();
                 reader.ThrowIfNotEmpty();
-                usages = new OidCollection();
+                OidCollection usages = new();
 
                 while (sequenceReader.HasData)
                 {
                     usages.Add(new Oid(sequenceReader.ReadObjectIdentifier(), null));
                 }
+
+                return usages;
             }
             catch (AsnContentException e)
             {

src/libraries/System.Security.Cryptography/tests/X509Certificates/ExtensionsTests/EnhancedKeyUsageTests.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Threading;
 using Test.Cryptography;
 using Xunit;
 
@@ -69,6 +70,39 @@ public static void CollectionPropertyIsolation()
             Assert.NotSame(e.EnhancedKeyUsages, e.EnhancedKeyUsages);
         }
 
+        [Fact]
+        public static void EnhancedKeyUsagesIsThreadSafe()
+        {
+            for (int i = 0; i < 10; i++)
+            {
+                X509EnhancedKeyUsageExtension ext = new(
+                    new AsnEncodedData("301606082b06010505070301060a2b0601040182370a0301".HexToByteArray()),
+                    critical: false);
+
+                Thread t1 = new(ThreadCallback);
+                Thread t2 = new(ThreadCallback);
+                ExtensionCount state1 = new(ext);
+                ExtensionCount state2 = new(ext);
+                t1.Start(state1);
+                t2.Start(state2);
+                t1.Join();
+                t2.Join();
+                Assert.Equal(2, state1.Count);
+                Assert.Equal(2, state2.Count);
+            }
+
+            static void ThreadCallback(object state)
+            {
+                ExtensionCount s = (ExtensionCount)state;
+                s.Count = s.Extensions.EnhancedKeyUsages.Count;
+            }
+        }
+
+        private sealed record ExtensionCount(X509EnhancedKeyUsageExtension Extensions)
+        {
+            public int Count { get; set; }
+        }
+
         private static void EncodeDecode(
             OidCollection usages,
             bool critical,