diff --git a/src/Localization/Abstractions/ref/Microsoft.Extensions.Localization.Abstractions.netstandard2.0.cs b/src/Localization/Abstractions/ref/Microsoft.Extensions.Localization.Abstractions.netstandard2.0.cs
index 3f9f422021..174cac28e5 100644
--- a/src/Localization/Abstractions/ref/Microsoft.Extensions.Localization.Abstractions.netstandard2.0.cs
+++ b/src/Localization/Abstractions/ref/Microsoft.Extensions.Localization.Abstractions.netstandard2.0.cs
@@ -16,7 +16,7 @@ namespace Microsoft.Extensions.Localization
         Microsoft.Extensions.Localization.IStringLocalizer Create(string baseName, string location);
         Microsoft.Extensions.Localization.IStringLocalizer Create(System.Type resourceSource);
     }
-    public partial interface IStringLocalizer<T> : Microsoft.Extensions.Localization.IStringLocalizer
+    public partial interface IStringLocalizer<out T> : Microsoft.Extensions.Localization.IStringLocalizer
     {
     }
     public partial class LocalizedString
diff --git a/src/Localization/Abstractions/src/IStringLocalizerOfT.cs b/src/Localization/Abstractions/src/IStringLocalizerOfT.cs
index 695678a900..bdc2a1c7b7 100644
--- a/src/Localization/Abstractions/src/IStringLocalizerOfT.cs
+++ b/src/Localization/Abstractions/src/IStringLocalizerOfT.cs
@@ -7,8 +7,8 @@ namespace Microsoft.Extensions.Localization
     /// Represents an <see cref="IStringLocalizer"/> that provides strings for <typeparamref name="T"/>.
     /// </summary>
     /// <typeparam name="T">The <see cref="System.Type"/> to provide strings for.</typeparam>
-    public interface IStringLocalizer<T> : IStringLocalizer
+    public interface IStringLocalizer<out T> : IStringLocalizer
     {
 
     }
-}
\ No newline at end of file
+}
diff --git a/src/Localization/Localization/test/StringLocalizerOfTTest.cs b/src/Localization/Localization/test/StringLocalizerOfTTest.cs
new file mode 100644
index 0000000000..ce06e74d1c
--- /dev/null
+++ b/src/Localization/Localization/test/StringLocalizerOfTTest.cs
@@ -0,0 +1,159 @@
+// 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.Globalization;
+using Moq;
+using Xunit;
+
+namespace Microsoft.Extensions.Localization
+{
+    public class StringLocalizerOfTTest
+    {
+        [Fact]
+        public void Constructor_ThrowsAnExceptionForNullFactory()
+        {
+            // Arrange, act and assert
+            var exception = Assert.Throws<ArgumentNullException>(
+                () => new StringLocalizer<object>(factory: null));
+
+            Assert.Equal("factory", exception.ParamName);
+        }
+
+        [Fact]
+        public void Constructor_ResolvesLocalizerFromFactory()
+        {
+            // Arrange
+            var factory = new Mock<IStringLocalizerFactory>();
+
+            // Act
+            _ = new StringLocalizer<object>(factory.Object);
+
+            // Assert
+            factory.Verify(mock => mock.Create(typeof(object)), Times.Once());
+        }
+
+        [Fact]
+        public void WithCulture_InvokesWithCultureFromInnerLocalizer()
+        {
+            // Arrange
+            var factory = new Mock<IStringLocalizerFactory>();
+            var innerLocalizer = new Mock<IStringLocalizer>();
+            factory.Setup(mock => mock.Create(typeof(object)))
+                .Returns(innerLocalizer.Object);
+
+            var localizer = new StringLocalizer<object>(factory.Object);
+
+            // Act
+#pragma warning disable CS0618
+            localizer.WithCulture(CultureInfo.GetCultureInfo("fr-FR"));
+#pragma warning restore CS0618
+
+            // Assert
+#pragma warning disable CS0618
+            innerLocalizer.Verify(mock => mock.WithCulture(CultureInfo.GetCultureInfo("fr-FR")), Times.Once());
+#pragma warning restore CS0618
+        }
+
+        [Fact]
+        public void Indexer_ThrowsAnExceptionForNullName()
+        {
+            // Arrange
+            var factory = new Mock<IStringLocalizerFactory>();
+            var innerLocalizer = new Mock<IStringLocalizer>();
+            factory.Setup(mock => mock.Create(typeof(object)))
+                .Returns(innerLocalizer.Object);
+
+            var localizer = new StringLocalizer<object>(factory.Object);
+
+            // Act and assert
+            var exception = Assert.Throws<ArgumentNullException>(() => localizer[name: null]);
+
+            Assert.Equal("name", exception.ParamName);
+        }
+
+        [Fact]
+        public void Indexer_InvokesIndexerFromInnerLocalizer()
+        {
+            // Arrange
+            var factory = new Mock<IStringLocalizerFactory>();
+            var innerLocalizer = new Mock<IStringLocalizer>();
+            factory.Setup(mock => mock.Create(typeof(object)))
+                .Returns(innerLocalizer.Object);
+
+            var localizer = new StringLocalizer<object>(factory.Object);
+
+            // Act
+            _ = localizer["Hello world"];
+
+            // Assert
+            innerLocalizer.Verify(mock => mock["Hello world"], Times.Once());
+        }
+
+        [Fact]
+        public void Indexer_ThrowsAnExceptionForNullName_WithArguments()
+        {
+            // Arrange
+            var factory = new Mock<IStringLocalizerFactory>();
+            var innerLocalizer = new Mock<IStringLocalizer>();
+            factory.Setup(mock => mock.Create(typeof(object)))
+                .Returns(innerLocalizer.Object);
+
+            var localizer = new StringLocalizer<object>(factory.Object);
+
+            // Act and assert
+            var exception = Assert.Throws<ArgumentNullException>(() => localizer[name: null]);
+
+            Assert.Equal("name", exception.ParamName);
+        }
+
+        [Fact]
+        public void Indexer_InvokesIndexerFromInnerLocalizer_WithArguments()
+        {
+            // Arrange
+            var factory = new Mock<IStringLocalizerFactory>();
+            var innerLocalizer = new Mock<IStringLocalizer>();
+            factory.Setup(mock => mock.Create(typeof(object)))
+                .Returns(innerLocalizer.Object);
+
+            var localizer = new StringLocalizer<object>(factory.Object);
+
+            // Act
+            _ = localizer["Welcome, {0}", "Bob"];
+
+            // Assert
+            innerLocalizer.Verify(mock => mock["Welcome, {0}", "Bob"], Times.Once());
+        }
+
+        [Fact]
+        public void GetAllStrings_InvokesGetAllStringsFromInnerLocalizer()
+        {
+            // Arrange
+            var factory = new Mock<IStringLocalizerFactory>();
+            var innerLocalizer = new Mock<IStringLocalizer>();
+            factory.Setup(mock => mock.Create(typeof(object)))
+                .Returns(innerLocalizer.Object);
+
+            var localizer = new StringLocalizer<object>(factory.Object);
+
+            // Act
+            localizer.GetAllStrings(includeParentCultures: true);
+
+            // Assert
+            innerLocalizer.Verify(mock => mock.GetAllStrings(true), Times.Once());
+        }
+
+        [Fact]
+        public void StringLocalizer_CanBeCastToBaseType()
+        {
+            // Arrange and act
+            IStringLocalizer<BaseType> localizer = new StringLocalizer<DerivedType>(Mock.Of<IStringLocalizerFactory>());
+
+            // Assert
+            Assert.NotNull(localizer);
+        }
+
+        private class BaseType { }
+        private class DerivedType : BaseType { }
+    }
+}