DebuggerDisplay Stopwatch

src/libraries/System.Private.CoreLib/src/System/Diagnostics/Stopwatch.cs

@@ -7,6 +7,7 @@ namespace System.Diagnostics
     // hardware supports it. Otherwise, the class will fall back to DateTime
     // and uses ticks as a measurement.
 
+    [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public partial class Stopwatch
     {
         private const long TicksPerMillisecond = 10000;
@@ -150,5 +151,7 @@ private long GetElapsedDateTimeTicks()
             // convert high resolution perf counter to DateTime ticks
             return unchecked((long)(GetRawElapsedTicks() * s_tickFrequency));
         }
+
+        private string DebuggerDisplay => $"{Elapsed} (IsRunning = {_isRunning})";
     }
 }

src/libraries/System.Runtime.Extensions/tests/System.Runtime.Extensions.Tests.csproj

@@ -82,6 +82,7 @@
 
     <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.GetHostName.cs" Condition="'$(TargetPlatformIdentifier)' == 'Unix' or '$(TargetPlatformIdentifier)' == 'Browser'" Link="Common\Interop\Unix\System.Native\Interop.GetHostName.cs" />
     <Compile Include="$(CommonPath)Interop\Unix\Interop.Libraries.cs" Link="Common\Interop\Unix\Interop.Libraries.cs" />
+    <Compile Include="$(CommonTestPath)System\Diagnostics\DebuggerAttributes.cs" Link="Common\System\Diagnostics\DebuggerAttributes.cs" />    
     <Compile Include="$(CommonTestPath)System\IO\PathFeatures.cs" Link="Common\System\IO\PathFeatures.cs" />
     <Compile Include="$(CommonTestPath)System\ShouldNotBeInvokedException.cs" Link="Common\System\ShouldNotBeInvokedException.cs" />
     <Compile Include="$(CommonTestPath)System\Security\Cryptography\ByteUtils.cs" Link="Common\System\Security\Cryptography\ByteUtils.cs" />

src/libraries/System.Runtime.Extensions/tests/System/Diagnostics/Stopwatch.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
+using System.Reflection;
 using System.Threading;
 using Xunit;
 
@@ -95,6 +96,27 @@ public static void StartNewAndRestart()
             }
         }
 
+        [Fact]
+        public static void DebuggerAttributesValid()
+        {
+            DebuggerAttributes.ValidateDebuggerDisplayReferences(new Stopwatch());
+
+            Stopwatch watch = new Stopwatch();
+            Assert.Equal("00:00:00 (IsRunning = False)", GetDebuggerDisplayProperty(watch));
+            watch.Start();
+            Thread.Sleep(10);
+            Assert.Contains("(IsRunning = True)", GetDebuggerDisplayProperty(watch));
+            Assert.DoesNotContain("00:00:00 ", GetDebuggerDisplayProperty(watch));
+            watch.Stop();
+            Assert.Contains("(IsRunning = False)", GetDebuggerDisplayProperty(watch));
+            Assert.DoesNotContain("00:00:00 ", GetDebuggerDisplayProperty(watch));
+
+            static string GetDebuggerDisplayProperty(Stopwatch value)
+            {
+                return (string)typeof(Stopwatch).GetProperty("DebuggerDisplay", BindingFlags.Instance | BindingFlags.NonPublic).GetValue(value);
+            }
+        }
+
         [OuterLoop("Sleeps for relatively long periods of time")]
         [Fact]
         public static void ElapsedMilliseconds_WithinExpectedWindow()