InMemoryDirectoryInfo breaking change

docs/core/compatibility/9.0.md

@@ -21,6 +21,7 @@ If you're migrating an app to .NET 9, the breaking changes listed here might aff
 |------------------------------------------------------------------------------------------|---------------------|--------------------|
 | [Creating type of array of System.Void not allowed](core-libraries/9.0/type-instance.md) | Behavioral change   | Preview 1          |
 | [Inline array struct size limit is enforced](core-libraries/9.0/inlinearray-size.md) | Behavioral change   | Preview 1          |
+| [InMemoryDirectoryInfo prepends rootDir to files](core-libraries/9.0/inmemorydirinfo-prepends-rootdir.md) | Behavioral change   | Preview 1          |
 | [RuntimeHelpers.GetSubArray returns different type](core-libraries/9.0/getsubarray-return.md) | Behavioral change   | Preview 1          |
 
 ## Networking

docs/core/compatibility/core-libraries/9.0/inmemorydirinfo-prepends-rootdir.md

@@ -0,0 +1,57 @@
+---
+title: "Breaking change: InMemoryDirectoryInfo prepends rootDir to files"
+description: Learn about the .NET 9 breaking change in core .NET libraries where InMemoryDirectoryInfo prepends the root directory to its file paths.
+ms.date: 02/12/2024
+---
+# InMemoryDirectoryInfo prepends rootDir to files
+
+<xref:Microsoft.Extensions.FileSystemGlobbing.InMemoryDirectoryInfo> now prepends the specified root directory to its collection of files.
+
+`InMemoryDirectoryInfo` is used by <xref:Microsoft.Extensions.FileSystemGlobbing.MatcherExtensions.Match%2A?displayProperty=nameWithType>, which enables the <xref:Microsoft.Extensions.FileSystemGlobbing.Matcher> to execute glob-matching patterns without accessing the disk.
+
+## Previous behavior
+
+Previously, relative paths in the `files` argument of the [constructor](xref:Microsoft.Extensions.FileSystemGlobbing.InMemoryDirectoryInfo.%23ctor(System.String,System.Collections.Generic.IEnumerable{System.String})) were prepended with the current working directory (CWD). This caused an unnecessary dependency on the CWD for a type that's supposed to work in-memory.
+
+## New behavior
+
+Starting in .NET 9, relative paths in the `files` argument of the constructor are prepended with the specified root directory.
+
+## Version introduced
+
+.NET 9 Preview 1
+
+## Type of breaking change
+
+This change is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+There were blocked scenarios with in-memory paths using a drive letter other than the one used by the current working directory. For example, see [dotnet/runtime issue 93107](https://github.com/dotnet/runtime/issues/93107).
+
+## Recommended action
+
+If you depended on the previous behavior, adjust your code to account for the files now being prepended with the root directory. For example:
+
+```diff
+// Since rootDir is also relative, it could've been used to filter the matching scope of `files`.
+-string rootDir = "dir1";
+// Now that's not possible; everything in `files` is under `root`.
++string rootDir = "root";
+string[] files = ["dir1/test.0", "dir1/subdir/test.1", "dir2/test.2"];
+
+-PatternMatchingResult result = new Matcher().AddInclude("**/*").Match(rootDir, files);
+// Adjust the pattern if you want to scope down to dir1.
++PatternMatchingResult result = new Matcher().AddInclude("dir1/**/*").Match(rootDir, files);
+Console.WriteLine(string.Join(", ", result.Files.Select(x => x.Path)));
+
+// Output:
+// dir1/test.0
+// dir1/subdir/test.1
+```
+
+## Affected APIs
+
+- <xref:Microsoft.Extensions.FileSystemGlobbing.InMemoryDirectoryInfo?displayProperty=fullName>
+- <xref:Microsoft.Extensions.FileSystemGlobbing.MatcherExtensions.Match(Microsoft.Extensions.FileSystemGlobbing.Matcher,System.String,System.String)?displayProperty=fullName>
+- <xref:Microsoft.Extensions.FileSystemGlobbing.MatcherExtensions.Match(Microsoft.Extensions.FileSystemGlobbing.Matcher,System.String,System.Collections.Generic.IEnumerable{System.String})?displayProperty=fullName>

docs/core/compatibility/toc.yml

@@ -14,6 +14,8 @@ items:
         href: core-libraries/9.0/type-instance.md
       - name: Inline array struct size limit is enforced
         href: core-libraries/9.0/inlinearray-size.md
+      - name: InMemoryDirectoryInfo prepends rootDir to files
+        href: core-libraries/9.0/inmemorydirinfo-prepends-rootdir.md
       - name: RuntimeHelpers.GetSubArray returns different type
         href: core-libraries/9.0/getsubarray-return.md
     - name: Networking
@@ -1142,6 +1144,8 @@ items:
         href: core-libraries/9.0/type-instance.md
       - name: Inline array struct size limit is enforced
         href: core-libraries/9.0/inlinearray-size.md
+      - name: InMemoryDirectoryInfo prepends rootDir to files
+        href: core-libraries/9.0/inmemorydirinfo-prepends-rootdir.md
       - name: RuntimeHelpers.GetSubArray returns different type
         href: core-libraries/9.0/getsubarray-return.md
     - name: .NET 8