Ensure all expected paths are logged on test failure (#65666)

* move duplicated logic to a method

* ensure all paths are logged, so we can diagnose #65601

* Apply suggestions from code review

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/libraries/System.IO.FileSystem.Watcher/tests/Utility/FileSystemWatcherTest.cs

@@ -8,6 +8,7 @@
 using Xunit;
 using Xunit.Sdk;
 using Xunit.Abstractions;
+using System.Linq;
 
 namespace System.IO.Tests
 {
@@ -39,17 +40,26 @@ public static (AutoResetEvent EventOccured, FileSystemEventHandler Handler) Watc
             FileSystemEventHandler changeHandler = (o, e) =>
             {
                 Assert.Equal(WatcherChangeTypes.Changed, e.ChangeType);
-                if (expectedPaths != null)
-                {
-                    Assert.Contains(Path.GetFullPath(e.FullPath), expectedPaths);
-                }
+                VerifyExpectedPaths(expectedPaths, e);
                 eventOccurred.Set();
             };
 
             watcher.Changed += changeHandler;
             return (eventOccurred, changeHandler);
         }
 
+        private static void VerifyExpectedPaths(string[] expectedPaths, FileSystemEventArgs e)
+        {
+            string fullPath = Path.GetFullPath(e.FullPath);
+            if (expectedPaths is not null && !expectedPaths.Contains(fullPath))
+            {
+                // Assert.Contains does not print a full content of collection which makes it hard to diagnose issues like #65601
+                throw new XunitException($"Expected path(s):{Environment.NewLine}"
+                    + string.Join(Environment.NewLine, expectedPaths)
+                    + $"{Environment.NewLine}Actual path: {fullPath}");
+            }
+        }
+
         /// <summary>
         /// Watches the Created WatcherChangeType and unblocks the returned AutoResetEvent when a
         /// Created event is thrown by the watcher.
@@ -67,18 +77,7 @@ public static (AutoResetEvent EventOccured, FileSystemEventHandler Handler) Watc
                 }
 
                 Assert.Equal(WatcherChangeTypes.Created, e.ChangeType);
-                if (expectedPaths != null)
-                {
-                    try
-                    {
-                        Assert.Contains(Path.GetFullPath(e.FullPath), expectedPaths);
-                    }
-                    catch (Exception ex)
-                    {
-                        _output?.WriteLine(ex.ToString());
-                        throw;
-                    }
-                }
+                VerifyExpectedPaths(expectedPaths, e);
 
                 eventOccurred.Set();
             };
@@ -102,18 +101,8 @@ public static (AutoResetEvent EventOccured, FileSystemEventHandler Handler) Watc
                     Assert.Equal(WatcherChangeTypes.Deleted, e.ChangeType);
                 }
 
-                if (expectedPaths != null)
-                {
-                    try
-                    {
-                        Assert.Contains(Path.GetFullPath(e.FullPath), expectedPaths);
-                    }
-                    catch (Exception ex)
-                    {
-                        _output?.WriteLine(ex.ToString());
-                        throw;
-                    }
-                }
+                VerifyExpectedPaths(expectedPaths, e);
+
                 eventOccurred.Set();
             };
 
@@ -137,18 +126,8 @@ public static (AutoResetEvent EventOccured, RenamedEventHandler Handler) WatchRe
                     Assert.Equal(WatcherChangeTypes.Renamed, e.ChangeType);
                 }
 
-                if (expectedPaths != null)
-                {
-                    try
-                    {
-                        Assert.Contains(Path.GetFullPath(e.FullPath), expectedPaths);
-                    }
-                    catch (Exception ex)
-                    {
-                        _output?.WriteLine(ex.ToString());
-                        throw;
-                    }
-                }
+                VerifyExpectedPaths(expectedPaths, e);
+
                 eventOccurred.Set();
             };