diff --git a/src/Microsoft.Framework.Localization/IResourceNamesCache.cs b/src/Microsoft.Framework.Localization/IResourceNamesCache.cs
new file mode 100644
index 0000000000..60b7c825cc
--- /dev/null
+++ b/src/Microsoft.Framework.Localization/IResourceNamesCache.cs
@@ -0,0 +1,22 @@
+// Copyright (c) .NET Foundation. All rights reserved. 
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information. 
+
+using System;
+using System.Collections.Generic;
+
+namespace Microsoft.Framework.Localization
+{
+    /// <summary>
+    /// Represents a cache of string names in resources.
+    /// </summary>
+    public interface IResourceNamesCache
+    {
+        /// <summary>
+        /// Adds a set of resource names to the cache by using the specified function, if the name does not already exist.
+        /// </summary>
+        /// <param name="name">The resource name to add string names for.</param>
+        /// <param name="valueFactory">The function used to generate the string names for the resource.</param>
+        /// <returns>The string names for the resource.</returns>
+        IList<string> GetOrAdd(string name, Func<string, IList<string>> valueFactory);
+    }
+}
diff --git a/src/Microsoft.Framework.Localization/ResourceManagerStringLocalizer.cs b/src/Microsoft.Framework.Localization/ResourceManagerStringLocalizer.cs
index e8b684599c..ee62f3db03 100644
--- a/src/Microsoft.Framework.Localization/ResourceManagerStringLocalizer.cs
+++ b/src/Microsoft.Framework.Localization/ResourceManagerStringLocalizer.cs
@@ -18,12 +18,10 @@ namespace Microsoft.Framework.Localization
     /// </summary>
     public class ResourceManagerStringLocalizer : IStringLocalizer
     {
-        private static readonly ConcurrentDictionary<string, IList<string>> _resourceNamesCache =
-            new ConcurrentDictionary<string, IList<string>>();
-
         private readonly ConcurrentDictionary<string, object> _missingManifestCache =
             new ConcurrentDictionary<string, object>();
 
+        private readonly IResourceNamesCache _resourceNamesCache;
         private readonly ResourceManager _resourceManager;
         private readonly AssemblyWrapper _resourceAssemblyWrapper;
         private readonly string _resourceBaseName;
@@ -34,11 +32,13 @@ namespace Microsoft.Framework.Localization
         /// <param name="resourceManager">The <see cref="System.Resources.ResourceManager"/> to read strings from.</param>
         /// <param name="resourceAssembly">The <see cref="Assembly"/> that contains the strings as embedded resources.</param>
         /// <param name="baseName">The base name of the embedded resource in the <see cref="Assembly"/> that contains the strings.</param>
+        /// <param name="resourceNamesCache">Cache of the list of strings for a given resource assembly name.</param>
         public ResourceManagerStringLocalizer(
             [NotNull] ResourceManager resourceManager,
             [NotNull] Assembly resourceAssembly,
-            [NotNull] string baseName)
-            : this(resourceManager, new AssemblyWrapper(resourceAssembly), baseName)
+            [NotNull] string baseName,
+            [NotNull] IResourceNamesCache resourceNamesCache)
+            : this(resourceManager, new AssemblyWrapper(resourceAssembly), baseName, resourceNamesCache)
         {
             
         }
@@ -49,11 +49,13 @@ namespace Microsoft.Framework.Localization
         public ResourceManagerStringLocalizer(
             [NotNull] ResourceManager resourceManager,
             [NotNull] AssemblyWrapper resourceAssemblyWrapper,
-            [NotNull] string baseName)
+            [NotNull] string baseName,
+            [NotNull] IResourceNamesCache resourceNamesCache)
         {
             _resourceAssemblyWrapper = resourceAssemblyWrapper;
             _resourceManager = resourceManager;
             _resourceBaseName = baseName;
+            _resourceNamesCache = resourceNamesCache;
         }
 
         /// <inheritdoc />
@@ -85,10 +87,16 @@ namespace Microsoft.Framework.Localization
         public IStringLocalizer WithCulture(CultureInfo culture)
         {
             return culture == null
-                ? new ResourceManagerStringLocalizer(_resourceManager, _resourceAssemblyWrapper, _resourceBaseName)
-                : new ResourceManagerWithCultureStringLocalizer(_resourceManager,
-                    _resourceAssemblyWrapper,
+                ? new ResourceManagerStringLocalizer(
+                    _resourceManager,
+                    _resourceAssemblyWrapper.Assembly,
                     _resourceBaseName,
+                    _resourceNamesCache)
+                : new ResourceManagerWithCultureStringLocalizer(
+                    _resourceManager,
+                    _resourceAssemblyWrapper.Assembly,
+                    _resourceBaseName,
+                    _resourceNamesCache,
                     culture);
         }
 
@@ -143,9 +151,6 @@ namespace Microsoft.Framework.Localization
             }
         }
 
-        // Internal to allow testing
-        internal static void ClearResourceNamesCache() => _resourceNamesCache.Clear();
-
         private IEnumerable<string> GetResourceNamesFromCultureHierarchy(CultureInfo startingCulture)
         {
             var currentCulture = startingCulture;
diff --git a/src/Microsoft.Framework.Localization/ResourceManagerStringLocalizerFactory.cs b/src/Microsoft.Framework.Localization/ResourceManagerStringLocalizerFactory.cs
index 92899e15a4..d0ebe781a1 100644
--- a/src/Microsoft.Framework.Localization/ResourceManagerStringLocalizerFactory.cs
+++ b/src/Microsoft.Framework.Localization/ResourceManagerStringLocalizerFactory.cs
@@ -5,7 +5,6 @@ using System;
 using System.Reflection;
 using System.Resources;
 using Microsoft.Framework.Internal;
-using Microsoft.Framework.Localization.Internal;
 using Microsoft.Framework.Runtime;
 
 namespace Microsoft.Framework.Localization
@@ -15,6 +14,8 @@ namespace Microsoft.Framework.Localization
     /// </summary>
     public class ResourceManagerStringLocalizerFactory : IStringLocalizerFactory
     {
+        private readonly IResourceNamesCache _resourceNamesCache = new ResourceNamesCache();
+
         private readonly IApplicationEnvironment _applicationEnvironment;
 
         /// <summary>
@@ -35,9 +36,13 @@ namespace Microsoft.Framework.Localization
         public IStringLocalizer Create([NotNull] Type resourceSource)
         {
             var typeInfo = resourceSource.GetTypeInfo();
-            var assembly = new AssemblyWrapper(typeInfo.Assembly);
+            var assembly = typeInfo.Assembly;
             var baseName = typeInfo.FullName;
-            return new ResourceManagerStringLocalizer(new ResourceManager(resourceSource), assembly, baseName);
+            return new ResourceManagerStringLocalizer(
+                new ResourceManager(resourceSource),
+                assembly,
+                baseName,
+                _resourceNamesCache);
         }
 
         /// <summary>
@@ -52,8 +57,9 @@ namespace Microsoft.Framework.Localization
 
             return new ResourceManagerStringLocalizer(
                 new ResourceManager(baseName, assembly),
-                new AssemblyWrapper(assembly),
-                baseName);
+                assembly,
+                baseName,
+                _resourceNamesCache);
         }
     }
 }
\ No newline at end of file
diff --git a/src/Microsoft.Framework.Localization/ResourceManagerWithCultureStringLocalizer.cs b/src/Microsoft.Framework.Localization/ResourceManagerWithCultureStringLocalizer.cs
index a451b10dbc..bc79cf838b 100644
--- a/src/Microsoft.Framework.Localization/ResourceManagerWithCultureStringLocalizer.cs
+++ b/src/Microsoft.Framework.Localization/ResourceManagerWithCultureStringLocalizer.cs
@@ -6,7 +6,6 @@ using System.Globalization;
 using System.Reflection;
 using System.Resources;
 using Microsoft.Framework.Internal;
-using Microsoft.Framework.Localization.Internal;
 
 namespace Microsoft.Framework.Localization
 {
@@ -24,26 +23,15 @@ namespace Microsoft.Framework.Localization
         /// <param name="resourceManager">The <see cref="System.Resources.ResourceManager"/> to read strings from.</param>
         /// <param name="resourceAssembly">The <see cref="Assembly"/> that contains the strings as embedded resources.</param>
         /// <param name="baseName">The base name of the embedded resource in the <see cref="Assembly"/> that contains the strings.</param>
+        /// <param name="resourceNamesCache">Cache of the list of strings for a given resource assembly name.</param>
         /// <param name="culture">The specific <see cref="CultureInfo"/> to use.</param>
         public ResourceManagerWithCultureStringLocalizer(
             [NotNull] ResourceManager resourceManager,
-            [NotNull] Assembly assembly,
+            [NotNull] Assembly resourceAssembly,
             [NotNull] string baseName,
+            [NotNull] IResourceNamesCache resourceNamesCache,
             [NotNull] CultureInfo culture)
-            : base(resourceManager, assembly, baseName)
-        {
-            _culture = culture;
-        }
-
-        /// <summary>
-        /// Intended for testing purposes only.
-        /// </summary>
-        public ResourceManagerWithCultureStringLocalizer(
-            [NotNull] ResourceManager resourceManager,
-            [NotNull] AssemblyWrapper assemblyWrapper,
-            [NotNull] string baseName,
-            [NotNull] CultureInfo culture)
-            : base(resourceManager, assemblyWrapper, baseName)
+            : base(resourceManager, resourceAssembly, baseName, resourceNamesCache)
         {
             _culture = culture;
         }
diff --git a/src/Microsoft.Framework.Localization/ResourceNamesCache.cs b/src/Microsoft.Framework.Localization/ResourceNamesCache.cs
new file mode 100644
index 0000000000..27ba70cf95
--- /dev/null
+++ b/src/Microsoft.Framework.Localization/ResourceNamesCache.cs
@@ -0,0 +1,23 @@
+// Copyright (c) .NET Foundation. All rights reserved. 
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information. 
+
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+
+namespace Microsoft.Framework.Localization
+{
+    /// <summary>
+    /// An implementation of <see cref="IResourceNamesCache"/> backed by a <see cref="ConcurrentDictionary{TKey, TValue}"/>.
+    /// </summary>
+    public class ResourceNamesCache : IResourceNamesCache
+    {
+        private readonly ConcurrentDictionary<string, IList<string>> _cache = new ConcurrentDictionary<string, IList<string>>();
+
+        /// <inheritdoc />
+        public IList<string> GetOrAdd(string name, Func<string, IList<string>> valueFactory)
+        {
+            return _cache.GetOrAdd(name, valueFactory);
+        }
+    }
+}
diff --git a/test/Microsoft.Framework.Localization.Test/ResourceManagerStringLocalizerTest.cs b/test/Microsoft.Framework.Localization.Test/ResourceManagerStringLocalizerTest.cs
index 410fc2c50f..92191c0bf8 100644
--- a/test/Microsoft.Framework.Localization.Test/ResourceManagerStringLocalizerTest.cs
+++ b/test/Microsoft.Framework.Localization.Test/ResourceManagerStringLocalizerTest.cs
@@ -17,12 +17,12 @@ namespace Microsoft.Framework.Localization.Test
         public void EnumeratorCachesCultureWalkForSameAssembly()
         {
             // Arrange
-            ResourceManagerStringLocalizer.ClearResourceNamesCache();
+            var resourceNamesCache = new ResourceNamesCache();
             var baseName = "test";
             var resourceAssembly = new TestAssemblyWrapper();
             var resourceManager = new TestResourceManager(baseName, resourceAssembly.Assembly);
-            var localizer1 = new ResourceManagerStringLocalizer(resourceManager, resourceAssembly, baseName);
-            var localizer2 = new ResourceManagerStringLocalizer(resourceManager, resourceAssembly, baseName);
+            var localizer1 = new ResourceManagerStringLocalizer(resourceManager, resourceAssembly, baseName, resourceNamesCache);
+            var localizer2 = new ResourceManagerStringLocalizer(resourceManager, resourceAssembly, baseName, resourceNamesCache);
 
             // Act
             for (int i = 0; i < 5; i++)
@@ -40,14 +40,14 @@ namespace Microsoft.Framework.Localization.Test
         public void EnumeratorCacheIsScopedByAssembly()
         {
             // Arrange
-            ResourceManagerStringLocalizer.ClearResourceNamesCache();
+            var resourceNamesCache = new ResourceNamesCache();
             var baseName = "test";
             var resourceAssembly1 = new TestAssemblyWrapper("Assembly1");
             var resourceAssembly2 = new TestAssemblyWrapper("Assembly2");
             var resourceManager1 = new TestResourceManager(baseName, resourceAssembly1.Assembly);
             var resourceManager2 = new TestResourceManager(baseName, resourceAssembly2.Assembly);
-            var localizer1 = new ResourceManagerStringLocalizer(resourceManager1, resourceAssembly1, baseName);
-            var localizer2 = new ResourceManagerStringLocalizer(resourceManager2, resourceAssembly2, baseName);
+            var localizer1 = new ResourceManagerStringLocalizer(resourceManager1, resourceAssembly1, baseName, resourceNamesCache);
+            var localizer2 = new ResourceManagerStringLocalizer(resourceManager2, resourceAssembly2, baseName, resourceNamesCache);
 
             // Act
             localizer1.ToList();