How to check if one path is a child of another path?

Question

How to check if one path is a child of another path?
Just checking for substring is not a way to go, because there can items such as . and .., etc

Answer 1

Unfortunately it's not as simple as StartsWith.

Here's a better answer, adapted from this duplicate question. I've made it an extension method for ease of use. Also using a brute-force catch as just about any method that accesses the file system can fail based on user permissions.

public static bool IsSubDirectoryOf(this string candidate, string other)
{
    var isChild = false;
    try
    {
        var candidateInfo = new DirectoryInfo(candidate);
        var otherInfo = new DirectoryInfo(other);

        while (candidateInfo.Parent != null)
        {
            if (candidateInfo.Parent.FullName == otherInfo.FullName)
            {
                isChild = true;
                break;
            }
            else candidateInfo = candidateInfo.Parent;
        }
    }
    catch (Exception error)
    {
        var message = String.Format("Unable to check directories {0} and {1}: {2}", candidate, other, error);
        Trace.WriteLine(message);
    }

    return isChild;
}

Answer 2

Any string-based solution is potentially subject to directory traversal attacks or correctness issues with things like trailing slashes. Unfortunately, the .NET Path class does not provide this functionality, however the Uri class does, in the form of Uri.IsBaseOf().

    Uri potentialBase = new Uri(@"c:\dir1\");

    Uri regular = new Uri(@"c:\dir1\dir2");

    Uri confusing = new Uri(@"c:\temp\..\dir1\dir2");

    Uri malicious = new Uri(@"c:\dir1\..\windows\system32\");

    Console.WriteLine(potentialBase.IsBaseOf(regular));   // True
    Console.WriteLine(potentialBase.IsBaseOf(confusing)); // True
    Console.WriteLine(potentialBase.IsBaseOf(malicious)); // False