Report a file passed as the root instead of crashing [patch]

FileDeduplicator.Test/FileScannerAndHasherTests.cs

@@ -68,6 +68,25 @@ public void ScanOfAMissingDirectoryReturnsNothing()
 		Assert.IsEmpty(files);
 	}
 
+	/// <summary>
+	/// A path to a file, rather than the folder holding it, must return an empty list rather than
+	/// throwing, so pasting a file path does not crash the tool.
+	/// </summary>
+	[TestMethod]
+	public void ScanOfAFileReturnsNothing()
+	{
+		// Arrange
+		using TempTree tree = new();
+		AbsoluteFilePath file = tree.Write("a/f.txt", "content");
+		AbsoluteDirectoryPath notADirectory = file.WeakString.As<AbsoluteDirectoryPath>();
+
+		// Act
+		IReadOnlyList<AbsoluteFilePath> files = FileScanner.ScanForFiles(notADirectory);
+
+		// Assert
+		Assert.IsEmpty(files);
+	}
+
 	/// <summary>
 	/// One unreadable directory must not abort the scan. Enumeration is lazy, so the refusal
 	/// arrives partway through the walk, after files elsewhere in the tree have already been found

FileDeduplicator.Test/VerbOutputTests.cs

@@ -36,6 +36,35 @@ private static void WriteOneGroup(TempTree tree, out AbsoluteFilePath keeper, ou
 		_ = tree.Write("unique.txt", "beta");
 	}
 
+	/// <summary>
+	/// Every verb given a file instead of a directory says so in one line and deletes nothing.
+	/// </summary>
+	[TestMethod]
+	public void EveryVerbReportsAFilePathAsNotADirectory()
+	{
+		// Arrange
+		using TempTree tree = new();
+		AbsoluteFilePath file = tree.Write("a/f.txt", "content");
+		string path = file.WeakString;
+
+		// Act
+		string[] outputs =
+		[
+			ConsoleCapture.Run(new Scan { PathString = path }),
+			ConsoleCapture.Run(new DryRun { PathString = path }),
+			ConsoleCapture.Run(new Stats { PathString = path }),
+			ConsoleCapture.Run(new Deduplicate { PathString = path }, "y"),
+		];
+
+		// Assert
+		foreach (string output in outputs)
+		{
+			Assert.Contains($"Not a directory: {path}", output);
+		}
+
+		Assert.IsTrue(TempTree.Exists(file), "The file named as the root must be left alone.");
+	}
+
 	/// <summary>
 	/// DryRun names the keeper and every other copy, totals them, and deletes nothing.
 	/// </summary>

FileDeduplicator/FileScanner.cs

@@ -44,7 +44,16 @@ internal static class FileScanner
 
 	internal static IReadOnlyList<AbsoluteFilePath> ScanForFiles(AbsoluteDirectoryPath path)
 	{
-		if (!path.Exists)
+		// path.Exists is also true for a file, which Directory.EnumerateFiles then rejects with an
+		// unhandled DirectoryNotFoundException. A path tab-completed one level too far is an easy
+		// mistake, so it gets the same one-line error as a path that is not there at all.
+		if (File.Exists(path.WeakString))
+		{
+			Console.WriteLine($"Not a directory: {path}");
+			return [];
+		}
+
+		if (!Directory.Exists(path.WeakString))
 		{
 			Console.WriteLine($"Directory not found: {path}");
 			return [];