diff --git a/src/Common/tests/System/Runtime/Serialization/Utils.cs b/src/Common/tests/System/Runtime/Serialization/Utils.cs
index 6861cb0360e8..4e314d01fd05 100644
--- a/src/Common/tests/System/Runtime/Serialization/Utils.cs
+++ b/src/Common/tests/System/Runtime/Serialization/Utils.cs
@@ -3,7 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
-using System.Collections.Generic;
+using System.Collections.Concurrent;
 using System.Diagnostics;
 using System.IO;
 using System.Text;
@@ -35,8 +35,8 @@ internal static void Equal<T>(T[] x, T[] y, Func<T, T, bool> f)
         }
     }
 
-    private static Dictionary<string, string> s_prefixToNamespaceDesk = new Dictionary<string, string>();
-    private static Dictionary<string, string> s_prefixToNamespaceCoreCLR = new Dictionary<string, string>();
+    private static ConcurrentDictionary<string, string> s_prefixToNamespaceDesk = new ConcurrentDictionary<string, string>();
+    private static ConcurrentDictionary<string, string> s_prefixToNamespaceCoreCLR = new ConcurrentDictionary<string, string>();
 
     internal struct CompareResult
     {
diff --git a/src/System.Private.DataContractSerialization/src/System/Runtime/Serialization/ExtensionDataReader.cs b/src/System.Private.DataContractSerialization/src/System/Runtime/Serialization/ExtensionDataReader.cs
index 62af8d097107..011b2bc35211 100644
--- a/src/System.Private.DataContractSerialization/src/System/Runtime/Serialization/ExtensionDataReader.cs
+++ b/src/System.Private.DataContractSerialization/src/System/Runtime/Serialization/ExtensionDataReader.cs
@@ -3,7 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Xml;
-using System.Collections.Generic;
+using System.Collections;
 
 namespace System.Runtime.Serialization
 {
@@ -37,20 +37,22 @@ private enum ExtensionDataNodeType
         private int _attributeCount;
         private int _attributeIndex;
 
+        private static object s_prefixLock = new object();
+
 #pragma warning disable 0649
         private XmlNodeReader _xmlNodeReader;
 #pragma warning restore 0649
 
         private XmlObjectSerializerReadContext _context;
 
-        private static Dictionary<string, string> s_nsToPrefixTable;
+        private static Hashtable s_nsToPrefixTable;
 
-        private static Dictionary<string, string> s_prefixToNsTable;
+        private static Hashtable s_prefixToNsTable;
 
         static ExtensionDataReader()
         {
-            s_nsToPrefixTable = new Dictionary<string, string>();
-            s_prefixToNsTable = new Dictionary<string, string>();
+            s_nsToPrefixTable = new Hashtable();
+            s_prefixToNsTable = new Hashtable();
             AddPrefix(Globals.XsiPrefix, Globals.SchemaInstanceNamespace);
             AddPrefix(Globals.SerPrefix, Globals.SerializationNamespace);
             AddPrefix(string.Empty, string.Empty);
@@ -205,10 +207,7 @@ public override string LookupNamespace(string prefix)
             if (IsXmlDataNode)
                 return _xmlNodeReader.LookupNamespace(prefix);
 
-            string ns;
-            if (!s_prefixToNsTable.TryGetValue(prefix, out ns))
-                return null;
-            return ns;
+            return (string)s_prefixToNsTable[prefix];
         }
 
         public override void Skip()
@@ -482,13 +481,14 @@ private ElementData GetNextElement()
 
         internal static string GetPrefix(string ns)
         {
-            string prefix;
             ns = ns ?? string.Empty;
-            if (!s_nsToPrefixTable.TryGetValue(ns, out prefix))
+            string prefix = (string)s_nsToPrefixTable[ns];
+            if (prefix == null) 
             {
-                lock (s_nsToPrefixTable)
+                lock (s_prefixLock)
                 {
-                    if (!s_nsToPrefixTable.TryGetValue(ns, out prefix))
+                    prefix = (string)s_nsToPrefixTable[ns];
+                    if (prefix == null) 
                     {
                         prefix = (ns == null || ns.Length == 0) ? string.Empty : "p" + s_nsToPrefixTable.Count;
                         AddPrefix(prefix, ns);
diff --git a/src/System.Runtime.Serialization.Xml/tests/SerializationTestTypes/DataContractResolverLibrary.cs b/src/System.Runtime.Serialization.Xml/tests/SerializationTestTypes/DataContractResolverLibrary.cs
index a3cba3554457..720b9d86fc26 100644
--- a/src/System.Runtime.Serialization.Xml/tests/SerializationTestTypes/DataContractResolverLibrary.cs
+++ b/src/System.Runtime.Serialization.Xml/tests/SerializationTestTypes/DataContractResolverLibrary.cs
@@ -3,7 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
-using System.Collections.Generic;
+using System.Collections.Concurrent;
 using System.Runtime.Serialization;
 using System.Xml;
 
@@ -13,7 +13,7 @@ namespace SerializationTestTypes
     public class PrimitiveTypeResolver : DataContractResolver
     {
         private readonly static string s_defaultNS = "http://www.default.com";
-        private readonly static Dictionary<string, Type> s_types = new Dictionary<string, Type>();
+        private readonly static ConcurrentDictionary<string, Type> s_types = new ConcurrentDictionary<string, Type>();
 
         public override bool TryResolveType(Type dcType, Type declaredType, DataContractResolver KTResolver, out XmlDictionaryString typeName, out XmlDictionaryString typeNamespace)
         {