fix: handle case-sensitive file overwrite on Windows and add tests (#1259)

This PR fixes an issue (#1140) where copying a file with the same name but different casing on Windows resulted in an "already in use" error. It aligns the behavior with the native file system.  

## Changes  
- Fixed file overwrite logic to properly handle case-insensitive paths on Windows.  
- Added Windows-specific checks to mimic `System.IO.File.Copy` `System.IO.File.Move` and `System.IO.File.Replace` behavior.  
- Implemented test cases to verify expected behavior for case-sensitive and case-insensitive file systems. 
- Ensured that the fix does not affect Linux behavior.  

## Testing  
- Verified that the fix works correctly on Windows by checking file existence before and after copying,moving and replacing file with different cases 
- Ran test cases on both Windows and Linux to ensure no unintended side effects.

Author: ana1250

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

@@ -48,7 +48,7 @@ public override void AppendAllBytes(string path, ReadOnlySpan<byte> bytes)
             AppendAllBytes(path, bytes.ToArray());
         }
 #endif
-        
+
     /// <inheritdoc />
     public override void AppendAllLines(string path, IEnumerable<string> contents)
     {
@@ -163,6 +163,11 @@ public override void Copy(string sourceFileName, string destFileName, bool overw
                 throw CommonExceptions.FileAlreadyExists(destFileName);
             }
 
+            if (string.Equals(sourceFileName, destFileName, StringComparison.OrdinalIgnoreCase) && XFS.IsWindowsPlatform())
+            {
+                throw CommonExceptions.ProcessCannotAccessFileInUse(destFileName);
+            }
+
             mockFileDataAccessor.RemoveFile(destFileName);
         }
 
@@ -522,30 +527,37 @@ public override void Move(string sourceFileName, string destFileName)
         mockFileDataAccessor.PathVerifier.IsLegalAbsoluteOrRelative(sourceFileName, nameof(sourceFileName));
         mockFileDataAccessor.PathVerifier.IsLegalAbsoluteOrRelative(destFileName, nameof(destFileName));
 
-        if (mockFileDataAccessor.GetFile(destFileName) != null)
-        {
-            if (mockFileDataAccessor.StringOperations.Equals(destFileName, sourceFileName))
-            {
-                return;
-            }
-            else
-            {
-                throw new IOException("A file can not be created if it already exists.");
-            }
-        }
-
         var sourceFile = mockFileDataAccessor.GetFile(sourceFileName);
 
         if (sourceFile == null)
         {
             throw CommonExceptions.FileNotFound(sourceFileName);
         }
+
         if (!sourceFile.AllowedFileShare.HasFlag(FileShare.Delete))
         {
             throw CommonExceptions.ProcessCannotAccessFileInUse();
         }
+
         VerifyDirectoryExists(destFileName);
 
+        if (mockFileDataAccessor.GetFile(destFileName) != null)
+        {
+            if (mockFileDataAccessor.StringOperations.Equals(destFileName, sourceFileName))
+            {
+                if (XFS.IsWindowsPlatform())
+                {
+                    mockFileDataAccessor.RemoveFile(sourceFileName);
+                    mockFileDataAccessor.AddFile(destFileName, mockFileDataAccessor.AdjustTimes(new MockFileData(sourceFile), TimeAdjustments.LastAccessTime), false);
+                }
+                return;
+            }
+            else
+            {
+                throw new IOException("A file can not be created if it already exists.");
+            }
+        }
+
         mockFileDataAccessor.RemoveFile(sourceFileName, false);
         mockFileDataAccessor.AddFile(destFileName, mockFileDataAccessor.AdjustTimes(new MockFileData(sourceFile), TimeAdjustments.LastAccessTime), false);
     }
@@ -822,6 +834,11 @@ public override void Replace(string sourceFileName, string destinationFileName,
             throw CommonExceptions.FileNotFound(destinationFileName);
         }
 
+        if (mockFileDataAccessor.StringOperations.Equals(sourceFileName, destinationFileName) && XFS.IsWindowsPlatform())
+        {
+            throw CommonExceptions.ProcessCannotAccessFileInUse();
+        }
+
         if (destinationBackupFileName != null)
         {
             Copy(destinationFileName, destinationBackupFileName, overwrite: true);
@@ -1066,7 +1083,7 @@ public override void WriteAllBytes(string path, byte[] bytes)
 
         mockFileDataAccessor.AddFile(path, mockFileDataAccessor.AdjustTimes(new MockFileData(bytes.ToArray()), TimeAdjustments.All));
     }
-        
+
 #if FEATURE_FILE_SPAN
         /// <inheritdoc cref="IFile.WriteAllBytes(string,ReadOnlySpan{byte})"/>
         public override void WriteAllBytes(string path, ReadOnlySpan<byte> bytes)

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

@@ -246,6 +246,22 @@ public async Task MockFile_Copy_ShouldThrowNotSupportedExceptionWhenSourcePathCo
         await That(action).Throws<NotSupportedException>();
     }
 
+    [Test]
+    [WindowsOnly(WindowsSpecifics.Drives)]
+    public async Task MockFile_Copy_ShouldThrowIOExceptionWhenOverwritingWithSameNameDifferentCase()
+    {
+        var fileSystem = new MockFileSystem();
+        string path = @"C:\Temp\file.txt";
+        string pathUpper = @"C:\Temp\FILE.TXT";
+
+        fileSystem.File.WriteAllText(path, "Hello");
+
+        void Act() => fileSystem.File.Copy(path, pathUpper, true);
+
+        await That(Act).Throws<IOException>()
+            .WithMessage($"The process cannot access the file '{pathUpper}' because it is being used by another process.");
+    }
+
     [Test]
     [WindowsOnly(WindowsSpecifics.Drives)]
     public async Task MockFile_Copy_ShouldThrowNotSupportedExceptionWhenSourcePathContainsInvalidDriveLetter()
@@ -417,4 +433,4 @@ public async Task MockFile_Copy_ShouldThrowIOExceptionForInvalidFileShare()
 
         await That(action).Throws<IOException>();
     }
-}
+}

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

@@ -249,6 +249,22 @@ public async Task MockFile_Move_ShouldThrowNotSupportedExceptionWhenDestinationP
         await That(action).Throws<NotSupportedException>();
     }
 
+    [Test]
+    [WindowsOnly(WindowsSpecifics.Drives)]
+    public async Task MockFile_Move_CaseOnlyRename_ShouldChangeCase()
+    {
+        var fileSystem = new MockFileSystem();
+        string sourceFilePath = @"c:\temp\demo.txt";
+        string destFilePath = @"c:\temp\DEMO.TXT";
+        string sourceFileContent = "content";
+        fileSystem.File.WriteAllText(sourceFilePath, sourceFileContent);
+
+        fileSystem.File.Move(sourceFilePath, destFilePath);
+
+        await That(fileSystem.File.Exists(destFilePath)).IsTrue();
+        await That(fileSystem.File.ReadAllText(destFilePath)).IsEqualTo(sourceFileContent);
+    }
+
     [Test]
     public async Task MockFile_Move_ShouldThrowArgumentExceptionWhenSourceIsEmpty_Message()
     {

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

@@ -547,7 +547,7 @@ public async Task MockFile_AppendText_CreatesNewFileForAppendToNonExistingFile()
         await That(file.TextContents).IsEqualTo("New too!");
         await That(filesystem.FileExists(filepath)).IsTrue();
     }
-        
+
 #if !NET9_0_OR_GREATER
     [Test]
     public void Serializable_works()
@@ -567,7 +567,7 @@ public void Serializable_works()
         Assert.Pass();
     }
 #endif
-        
+
 #if !NET9_0_OR_GREATER
     [Test]
     public async Task Serializable_can_deserialize()
@@ -670,7 +670,7 @@ public async Task MockFile_Replace_ShouldCreateBackup()
         fileSystem.File.Replace(path1, path2, path3);
 
         await That(fileSystem.File.ReadAllText(path3)).IsEqualTo("2");
-    } 
+    }
 
     [Test]
     public async Task MockFile_Replace_ShouldThrowIfDirectoryOfBackupPathDoesNotExist()
@@ -730,4 +730,20 @@ public async Task MockFile_OpenRead_ShouldReturnReadOnlyStream()
         await That(stream.CanWrite).IsFalse();
         await That(() => stream.WriteByte(0)).Throws<NotSupportedException>();
     }
+
+    [Test]
+    [WindowsOnly(WindowsSpecifics.Drives)]
+    public async Task MockFile_Replace_SourceAndDestinationDifferOnlyInCasing_ShouldThrowIOException()
+    {
+        var fileSystem = new MockFileSystem();
+        string sourceFilePath = @"c:\temp\demo.txt";
+        string destFilePath = @"c:\temp\DEMO.txt";
+        string fileContent = "content";
+        fileSystem.File.WriteAllText(sourceFilePath, fileContent);
+
+        void Act() => fileSystem.File.Replace(sourceFilePath, destFilePath, null, true);
+
+        await That(Act).Throws<IOException>()
+            .HasMessage("The process cannot access the file because it is being used by another process.");
+    }
 }