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Time fields on symbolic links

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When changes are made to the following time-related fields on a symbolic link ("symlink"), the updates now affect the symlink itself and not the target:

Previous behavior

Previously, updating any of the time-related fields on a symlink affected the fields of the symlink's target.

Consider the following program that prints the various time field values on a file and its symbolic link, updates the symlink's time field values to 1 day later, and then reprints the time field values on both the file and symlink.

string filename = "file";
string linkname = "link";

// Create a file and symlink.
File.Create(filename).Dispose();
File.CreateSymbolicLink(linkname, filename);

Console.WriteLine("Before update:");
PrintMetadata(filename);
PrintMetadata(linkname);

UpdateMetadata(linkname);

Console.WriteLine("\nAfter update:");
PrintMetadata(filename);
PrintMetadata(linkname);

static void UpdateMetadata(string filename)
{
    DateTime tomorrow = DateTime.Now.AddDays(1);
    
    File.SetCreationTime(filename, tomorrow);
    File.SetLastAccessTime(filename, tomorrow);
    File.SetLastWriteTime(filename, tomorrow);
    File.SetAttributes(filename, File.GetAttributes(filename) | FileAttributes.Offline);
}

static void PrintMetadata(string filename)
{
    Console.WriteLine($"---{filename}---");
    Console.WriteLine("Creation:\t" + File.GetCreationTime(filename));
    Console.WriteLine("Last access:\t" + File.GetLastAccessTime(filename));
    Console.WriteLine("Last write:\t" + File.GetLastWriteTime(filename));
    Console.WriteLine("Attributes:\t" + File.GetAttributes(filename));
}

Previously, after updating the values on the symlink, only the target file's time fields were updated. The output of the preceding program was as follows:

Before update:
---file---
Creation:       9/29/2022 10:35:40 AM
Last access:    9/29/2022 10:35:40 AM
Last write:     9/29/2022 10:35:40 AM
Attributes:     Archive
---link---
Creation:       9/29/2022 10:35:40 AM
Last access:    9/29/2022 10:35:40 AM
Last write:     9/29/2022 10:35:40 AM
Attributes:     Archive, ReparsePoint

After update:
---file---
Creation:       9/30/2022 10:35:40 AM
Last access:    9/30/2022 10:35:40 AM
Last write:     9/30/2022 10:35:40 AM
Attributes:     Archive
---link---
Creation:       9/29/2022 10:35:40 AM
Last access:    9/29/2022 10:35:40 AM
Last write:     9/29/2022 10:35:40 AM
Attributes:     Archive, ReparsePoint, Offline

New behavior

Starting in .NET 7, updating any of the time-related fields on a symlink affects the fields of the symlink itself and not the target file.

The output of the program shown in the Previous behavior section is as follows:

Before update:
---file---
Creation:       9/29/2022 10:33:39 AM
Last access:    9/29/2022 10:33:39 AM
Last write:     9/29/2022 10:33:39 AM
Attributes:     Archive
---link---
Creation:       9/29/2022 10:33:39 AM
Last access:    9/29/2022 10:33:39 AM
Last write:     9/29/2022 10:33:39 AM
Attributes:     Archive, ReparsePoint

After update:
---file---
Creation:       9/29/2022 10:33:39 AM
Last access:    9/29/2022 10:33:39 AM
Last write:     9/29/2022 10:33:39 AM
Attributes:     Archive
---link---
Creation:       9/30/2022 10:33:39 AM
Last access:    9/30/2022 10:33:39 AM
Last write:     9/30/2022 10:33:39 AM
Attributes:     Archive, ReparsePoint, Offline

Version introduced

.NET 7 Preview 1

Type of breaking change

This change can affect binary compatibility.

Reason for change

The previous behavior was unexpected and undesirable in some cases:

  • It was inconsistent with the behavior of the properties and methods that get the same fields.
  • It was also impossible to actually update the fields in the symlink itself using .NET APIs.

If you were relying on this behavior to set values on the symlink's target, setting one of the *Time fields in a symlink will no longer affect the target. You can use the new symbolic link APIs to obtain the target of a symlink and then update that file system object instead.

FileSystemInfo? targetInfo = linkInfo.ResolveLinkTarget(returnFinalTarget: true);
if (targetInfo  != null)
{
    // Update the properties accordingly.
    targetInfo.LastWriteTime = DateTime.Now;
}

Affected APIs