Path.GetDirectoryName

This C# method finds a directory name from a path. It handles path formats in a reliable way. We look at this method from System.IO.

We look inside GetDirectoryName and develop an optimized version. It is possible to remove certain features from its implementation.

Example program

First, there are many advantages to the Path static class in the base class library, but it also has drawbacks. It checks extensively for errors.

using System;
using System.IO;

class Program
{
    static void Main()
    {
        // Example path.
        string path = @"C:\Users\Sam\Documents\Test.txt";

        // Get the directory name.
        Console.WriteLine(Path.GetDirectoryName(path));
    }
}
C:\Users\Sam\Documents

Path optimization

Is the GetDirectoryName() method fully optimized in .NET 7 in 2022? We test whether we should still use a custom string method to check some paths.

using System;
using System.Diagnostics;
using System.IO;

class Program
{
    static string GetDirectoryName(string value)
    {
        // This char may need to be changed.
        var last = value.LastIndexOf('/');
        if (last == -1)
        {
            return string.Empty;
        }
        return value.Substring(0, last);
    }

    const int _max = 1000000;
    static void Main()
    {
        Console.WriteLine(Path.GetDirectoryName( @"/Users/sam/Documents/Test.txt"));
        Console.WriteLine(GetDirectoryName( @"/Users/sam/Documents/Test.txt"));

        var s1 = Stopwatch.StartNew();
        // Version 1: use Path function.
        for (int i = 0; i < _max; i++)
        {
            var result = Path.GetDirectoryName( @"/Users/sam/Documents/Test.txt");
        }
        s1.Stop();
        var s2 = Stopwatch.StartNew();
        // Version 2: use simple custom function.
        for (int i = 0; i < _max; i++)
        {
            var result = GetDirectoryName( @"/Users/sam/Documents/Test.txt");
        }
        s2.Stop();
        Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
        Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
    }
}
/Users/sam/Documents
/Users/sam/Documents
29.52 ns    Path.GetDirectoryName
11.74 ns    GetDirectoryName

Notes, internals

Path tries to detect all invalid paths and fix them if necessary. However, these automatic fixes can be slow—they involve complicated internal routines.

We looked into Path.GetDirectoryName and observed its input and output. This method returns a string indicating the containing directory.