Add a few IdnMapping tests (#28797)

Validate that we get the same string object back when we expect to.

2 files changed, 22 insertions, 0 deletions

diff --git a/src/System.Globalization.Extensions/tests/IdnMapping/IdnMappingGetAsciiTests.cs b/src/System.Globalization.Extensions/tests/IdnMapping/IdnMappingGetAsciiTests.cs
index 977f74d0f0..020f0ef947 100644
--- a/src/System.Globalization.Extensions/tests/IdnMapping/IdnMappingGetAsciiTests.cs
+++ b/src/System.Globalization.Extensions/tests/IdnMapping/IdnMappingGetAsciiTests.cs
@@ -84,6 +84,17 @@ namespace System.Globalization.Tests
             Assert.Equal(expected, new IdnMapping().GetAscii(unicode, index, count));
         }
 
+        [SkipOnTargetFramework(~TargetFrameworkMonikers.Netcoreapp, "Optimization in .NET Core")]
+        [Theory]
+        [InlineData("www.microsoft.com")]
+        [InlineData("bing.com")]
+        public void GetAscii_NoTranslationNeeded_ResultIsSameObjectAsInput(string input)
+        {
+            Assert.Same(input, new IdnMapping().GetAscii(input));
+            Assert.NotSame(input, new IdnMapping().GetAscii(input.Substring(1)));
+            Assert.NotSame(input, new IdnMapping().GetAscii(input.Substring(0, input.Length - 1)));
+        }
+
         [Fact]
         public void TestGetAsciiWithDot()
         {
diff --git a/src/System.Globalization.Extensions/tests/IdnMapping/IdnMappingGetUnicodeTests.cs b/src/System.Globalization.Extensions/tests/IdnMapping/IdnMappingGetUnicodeTests.cs
index 142d30c535..3022b3a869 100644
--- a/src/System.Globalization.Extensions/tests/IdnMapping/IdnMappingGetUnicodeTests.cs
+++ b/src/System.Globalization.Extensions/tests/IdnMapping/IdnMappingGetUnicodeTests.cs
@@ -59,6 +59,17 @@ namespace System.Globalization.Tests
             Assert.Equal(expected, new IdnMapping().GetUnicode(ascii, index, count));
         }
 
+        [SkipOnTargetFramework(~TargetFrameworkMonikers.Netcoreapp, "Optimization in .NET Core")]
+        [Theory]
+        [InlineData("www.microsoft.com")]
+        [InlineData("bing.com")]
+        public void GetUnicode_NoTranslationNeeded_ResultIsSameObjectAsInput(string input)
+        {
+            Assert.Same(input, new IdnMapping().GetUnicode(input));
+            Assert.NotSame(input, new IdnMapping().GetUnicode(input.Substring(1)));
+            Assert.NotSame(input, new IdnMapping().GetUnicode(input.Substring(0, input.Length - 1)));
+        }
+
         public static IEnumerable<object[]> GetUnicode_Invalid_TestData()
         {
             // Ascii is null