Improve exception message when passing directory to ZipFile methods (#121917)

Fixes an issue where passing a directory path to `ZipFile` methods
resulted in an unclear `UnauthorizedAccessException` without explaining
that the problem was passing a directory instead of a file.

Modified `GetFileStreamForOpen()` to catch `UnauthorizedAccessException`
when `FileStream` fails to open a directory, and re-throw with a clearer
error message that explicitly states the path is a directory.

Fixes #100720

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: iremyux

src/libraries/System.IO.Compression.ZipFile/src/Resources/Strings.resx

@@ -100,6 +100,9 @@
   <data name="UnauthorizedAccess_IODenied_Path" xml:space="preserve">
     <value>Access to the path '{0}' is denied.</value>
   </data>
+  <data name="IO_DirectoryNotAllowed" xml:space="preserve">
+    <value>The specified path '{0}' is a directory, which is not allowed in this operation.</value>
+  </data>
   <data name="ZipUnsupportedFile" xml:space="preserve">
     <value>The file type of '{0}' is not supported for zip archiving.</value>
   </data>

src/libraries/System.IO.Compression.ZipFile/src/System/IO/Compression/ZipFile.Create.cs

@@ -463,7 +463,12 @@ private static void CreateZipArchiveFromDirectory(string sourceDirectoryName, Zi
 
         private static FileStream GetFileStreamForOpen(ZipArchiveMode mode, string archiveFileName, bool useAsync)
         {
-            // Relies on FileStream's ctor for checking of archiveFileName
+            // Check if the path is a directory before attempting to open,
+            // to match the UnauthorizedAccessException thrown by FileStream's ctor.
+            if (Directory.Exists(archiveFileName))
+            {
+                throw new UnauthorizedAccessException(SR.Format(SR.IO_DirectoryNotAllowed, archiveFileName));
+            }
 
             (FileMode fileMode, FileAccess access, FileShare fileShare) = mode switch
             {
@@ -473,6 +478,7 @@ private static FileStream GetFileStreamForOpen(ZipArchiveMode mode, string archi
                 _ => throw new ArgumentOutOfRangeException(nameof(mode)),
             };
 
+            // Relies on FileStream's ctor for checking of archiveFileName
             return new FileStream(archiveFileName, fileMode, access, fileShare, bufferSize: FileStreamBufferSize, useAsync);
         }
 

src/libraries/System.IO.Compression.ZipFile/tests/ZipFile.Extract.cs

@@ -42,6 +42,17 @@ public async Task ExtractToDirectoryNull(bool async)
             await AssertExtensions.ThrowsAsync<ArgumentNullException>("sourceArchiveFileName", () => CallZipFileExtractToDirectory(async, sourceArchiveFileName: null, GetTestFilePath()));
         }
 
+        [Theory]
+        [MemberData(nameof(Get_Booleans_Data))]
+        public async Task ExtractToDirectory_PassDirectoryAsArchiveFile_ThrowsUnauthorizedAccessException(bool async)
+        {
+            string directoryPath = GetTestFilePath();
+            Directory.CreateDirectory(directoryPath);
+
+            await Assert.ThrowsAsync<UnauthorizedAccessException>(() =>
+                CallZipFileExtractToDirectory(async, directoryPath, GetTestFilePath()));
+        }
+
         [Theory]
         [MemberData(nameof(Get_Booleans_Data))]
         public async Task ExtractToDirectoryUnicode(bool async)

src/libraries/System.IO.Compression.ZipFile/tests/ZipFile.Open.cs

@@ -18,6 +18,30 @@ public Task InvalidConstructors(bool async)
         return Assert.ThrowsAsync<ArgumentOutOfRangeException>(() => CallZipFileOpen(async, "bad file", (ZipArchiveMode)(10)));
     }
 
+    [Fact]
+    public void Open_PassDirectory_ThrowsUnauthorizedAccessException()
+    {
+        string directoryPath = GetTestFilePath();
+        Directory.CreateDirectory(directoryPath);
+
+        Assert.Throws<UnauthorizedAccessException>(() => ZipFile.Open(directoryPath, ZipArchiveMode.Read));
+        Assert.Throws<UnauthorizedAccessException>(() => ZipFile.OpenRead(directoryPath));
+        Assert.Throws<UnauthorizedAccessException>(() => ZipFile.Open(directoryPath, ZipArchiveMode.Create));
+        Assert.Throws<UnauthorizedAccessException>(() => ZipFile.Open(directoryPath, ZipArchiveMode.Update));
+    }
+
+    [Fact]
+    public async Task OpenAsync_PassDirectory_ThrowsUnauthorizedAccessException()
+    {
+        string directoryPath = GetTestFilePath();
+        Directory.CreateDirectory(directoryPath);
+
+        await Assert.ThrowsAsync<UnauthorizedAccessException>(() => ZipFile.OpenAsync(directoryPath, ZipArchiveMode.Read, default));
+        await Assert.ThrowsAsync<UnauthorizedAccessException>(() => ZipFile.OpenReadAsync(directoryPath, default));
+        await Assert.ThrowsAsync<UnauthorizedAccessException>(() => ZipFile.OpenAsync(directoryPath, ZipArchiveMode.Create, default));
+        await Assert.ThrowsAsync<UnauthorizedAccessException>(() => ZipFile.OpenAsync(directoryPath, ZipArchiveMode.Update, default));
+    }
+
     [Fact]
     public void InvalidFiles()
     {