fix: MockFileSystem's File.WriteAllText, File.ReadAllText, Directory.GetFiles, Directory.Create fail with \\?\C:\foo style paths

src/TestableIO.System.IO.Abstractions.TestingHelpers/PathVerifier.cs

@@ -1,4 +1,5 @@
-using System.Linq;
+using System;
+using System.Linq;
 
 namespace System.IO.Abstractions.TestingHelpers;
 
@@ -15,6 +16,10 @@ public class PathVerifier
     private static readonly char[] AdditionalInvalidPathChars = { '*', '?' };
     private readonly IMockFileDataAccessor _mockFileDataAccessor;
 
+    // Windows supports extended-length paths with a `\\?\` prefix, to work around low path length limits.
+    // Ref: https://learn.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation?tabs=registry
+    private const string WINDOWS_EXTENDED_LENGTH_PATH_PREFIX = @"\\?\";
+
     /// <summary>
     /// Creates a new verifier instance.
     /// </summary>
@@ -64,6 +69,12 @@ public void IsLegalAbsoluteOrRelative(string path, string paramName)
 
     private static bool IsValidUseOfVolumeSeparatorChar(string path)
     {
+        if (XFS.IsWindowsPlatform() && path.StartsWith(WINDOWS_EXTENDED_LENGTH_PATH_PREFIX))
+        {
+            // Skip over the `\\?\` prefix if there is one.
+            path = path.Substring(WINDOWS_EXTENDED_LENGTH_PATH_PREFIX.Length);
+        }
+
         var lastVolSepIndex = path.LastIndexOf(Path.VolumeSeparatorChar);
         return lastVolSepIndex == -1 || lastVolSepIndex == 1 && char.IsLetter(path[0]);
     }
@@ -97,6 +108,14 @@ public bool HasIllegalCharacters(string path, bool checkAdditional)
 
         if (checkAdditional)
         {
+            // AdditionalInvalidPathChars includes '?', but this character is allowed in extended-length
+            // windows path prefixes (`\\?\`). If we're dealing with such a path, check for invalid
+            // characters after the prefix.
+            if (XFS.IsWindowsPlatform() && path.StartsWith(WINDOWS_EXTENDED_LENGTH_PATH_PREFIX))
+            {
+                path = path.Substring(WINDOWS_EXTENDED_LENGTH_PATH_PREFIX.Length);
+            }
+
             return path.IndexOfAny(invalidPathChars.Concat(AdditionalInvalidPathChars).ToArray()) >= 0;
         }
 
@@ -164,4 +183,4 @@ public bool TryNormalizeDriveName(string name, out string result)
         result = name;
         return true;
     }
-}
+}

tests/TestableIO.System.IO.Abstractions.TestingHelpers.Tests/MockDirectoryTests.cs

@@ -747,6 +747,25 @@ public async Task MockDirectory_CreateDirectory_ShouldThrowIfIllegalCharacterInP
         await That(createDelegate).Throws<ArgumentException>();
     }
 
+    [Test]
+    [WindowsOnly(WindowsSpecifics.UNCPaths)]
+    public async Task MockDirectory_CreateDirectory_ShouldSupportExtendedLengthPaths()
+    {
+        // Arrange
+        var fileSystem = new MockFileSystem();
+
+        // Act
+        var directoryInfo = fileSystem.Directory.CreateDirectory(XFS.Path(@"\\?\c:\bar"));
+        fileSystem.File.WriteAllText(@"\\?\c:\bar\grok.txt", "hello world\n");
+
+        // Assert
+        await That(fileSystem.Directory.Exists(XFS.Path(@"\\?\c:\bar"))).IsTrue();
+        await That(directoryInfo.FullName).IsEqualTo(@"\\?\c:\bar");
+        await That(fileSystem.File.ReadAllText(@"\\?\c:\bar\grok.txt")).IsEqualTo("hello world\n");
+        await That(fileSystem.Directory.GetFiles(@"\\?\c:\bar")).HasSingle()
+            .Which.IsEqualTo(@"\\?\c:\bar\grok.txt");
+    }
+
     // Issue #210
     [Test]
     public async Task MockDirectory_CreateDirectory_ShouldIgnoreExistingDirectoryRegardlessOfTrailingSlash()
@@ -2219,4 +2238,4 @@ public static void MockDirectory_Move_ShouldNotThrowException_InWindows_When_Sou
         // Act & Assert
         Assert.DoesNotThrow(() => mockFs.Directory.Move(src, dest));
     }
-}
+}