File.Copy() and Symbolic Links

Question

I want to copy a file to a new file name. Sometimes the source file might be a symbolic (file) link, created with

mklink C:\MyPath\ThisIsASymbolicLink.xml C:\MyPath\ThisIsTheOriginal.xml

I'm using this code:

string from = @"C:\MyPath\ThisIsASymbolicLink.xml";
string to = @"C:\MyPath\WantCopyOfOriginalFileHere.xml";
File.Copy(from, to, true);

However, I receive an IOException

The name of the file cannot be resolved by the system.

when the from file is really a symbolic link.

How can I code for the cases where the source file might be a real file, or a symbolic link to a file?

Answer 1

Expanding on this blog post, I created extension methods that take a DirectoryInfo or FileInfo that can refer to either an original or a symbolic link, and return a string indicating the fully qualified path name of the original file.

App Code

The application code is modified as follows:

// Works whether or not file is a symbolic link
string from = 
    new FileInfo(@"C:\MyPath\ThisIsASymbolicLink.xml").GetSymbolicLinkTarget();

Extension Method Code

    private const int FILE_SHARE_READ = 1;
    private const int FILE_SHARE_WRITE = 2;

    private const int CREATION_DISPOSITION_OPEN_EXISTING = 3;

    private const int FILE_FLAG_BACKUP_SEMANTICS = 0x02000000;

    // http://msdn.microsoft.com/en-us/library/aa364962%28VS.85%29.aspx
    [DllImport("kernel32.dll", EntryPoint = "GetFinalPathNameByHandleW", CharSet = CharSet.Unicode, SetLastError = true)]
    public static extern int GetFinalPathNameByHandle(IntPtr handle, [In, Out] StringBuilder path, int bufLen, int flags);

    // http://msdn.microsoft.com/en-us/library/aa363858(VS.85).aspx
    [DllImport("kernel32.dll", EntryPoint = "CreateFileW", CharSet = CharSet.Unicode, SetLastError = true)]
    public static extern SafeFileHandle CreateFile(string lpFileName, int dwDesiredAccess, int dwShareMode,
    IntPtr SecurityAttributes, int dwCreationDisposition, int dwFlagsAndAttributes, IntPtr hTemplateFile);

    public static string GetSymbolicLinkTarget(this FileSystemInfo symlink)
    {
        using (SafeFileHandle fileHandle = CreateFile(symlink.FullName, 0, 2, System.IntPtr.Zero, CREATION_DISPOSITION_OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, System.IntPtr.Zero))
        {
            if (fileHandle.IsInvalid)
                throw new Win32Exception(Marshal.GetLastWin32Error());

            StringBuilder path = new StringBuilder(512);
            int size = GetFinalPathNameByHandle(fileHandle.DangerousGetHandle(), path, path.Capacity, 0);
            if (size < 0)
                throw new Win32Exception(Marshal.GetLastWin32Error());
            // The remarks section of GetFinalPathNameByHandle mentions the return being prefixed with "\\?\"
            // More information about "\\?\" here -> http://msdn.microsoft.com/en-us/library/aa365247(v=VS.85).aspx
            if (path[0] == '\\' && path[1] == '\\' && path[2] == '?' && path[3] == '\\')
                return path.ToString().Substring(4);
            else
                return path.ToString();
        }
    }

Unit Tests

[TestClass]
public class SymlinkTest
{
    [TestInitialize]
    public void SetupFiles()
    {
        if (!File.Exists(@"C:\Temp\SymlinkUnitTest\Original.txt")) throw new Exception("Run Symlinksetup.bat as Admin to create test data.");
    }

    [TestMethod]
    public void OrdinaryFile()
    {
        string file = @"C:\Temp\SymlinkUnitTest\Original.txt";
        string actual = new FileInfo(file).GetSymbolicLinkTarget();
        Assert.IsTrue(actual.EndsWith(@"SymlinkUnitTest\Original.txt"));
    }

    [TestMethod]
    public void FileSymlink()
    {
        string file = @"C:\Temp\SymlinkUnitTest\Symlink.txt";
        string actual = new FileInfo(file).GetSymbolicLinkTarget();
        Assert.IsTrue(actual.EndsWith(@"SymlinkUnitTest\Original.txt"));
    }

    [TestMethod]
    public void OrdinaryDirectory()
    {
        string dir = @"C:\Temp\SymlinkUnitTest";
        string actual = new DirectoryInfo(dir).GetSymbolicLinkTarget();
        Assert.IsTrue(actual.EndsWith(@"SymlinkUnitTest"));
    }

    [TestMethod]
    public void DirectorySymlink()
    {
        string dir = @"C:\Temp\SymlinkUnitTest";
        string actual = new DirectoryInfo(dir).GetSymbolicLinkTarget();
        Assert.IsTrue(actual.EndsWith(@"SymlinkUnitTest"));
    }
}

Batch File to Create Unit Test Data

Must be run as Administrator... a requirement of mklink.

@Echo Off
Echo Must be run as Administrator (due to mklink)
mkdir C:\Temp
mkdir C:\Temp\SymlinkUnitTest
c:
cd C:\Temp\SymlinkUnitTest

echo Original File>Original.txt
mklink Symlink.txt Original.txt

mklink /D C:\Temp\SymlinkUnitTest\SymDir C:\Temp\SymlinkUnitTest