Convert an absolute path to a relative path.

PathUtil.cs

using System;
using System.IO;
using System.Text;

public static class PathUtil
{
    public static string AbsoluteToRelative(DirectoryInfo baseDir, FileInfo target)
    {
        var baseParts = baseDir.FullName.Replace('/', '\\').TrimEnd('\\').Split('\\');
        var targetParts = target.FullName.Replace('/', '\\').Split('\\');

        var relativePathBuilder = new StringBuilder();

        // find common root depth
        var depth = 0;
        for (; depth < Math.Min(baseParts.Length, targetParts.Length); depth++)
        {
            if (baseParts[depth] != targetParts[depth]) break;
        }

        // move upward from base dir
        for (var i = 0; i < baseParts.Length - depth; i++)
        {
            relativePathBuilder.Append(@"..\");
        }

        // move downward to target
        for (var i = depth; i < targetParts.Length; i++)
        {
            relativePathBuilder.Append(targetParts[i]);
            if (i + 1 != targetParts.Length)
            {
                relativePathBuilder.Append(@"\");
            }
        }

        return relativePathBuilder.ToString();
    }
}

PathUtilTest.cs

using Microsoft.VisualStudio.TestTools.UnitTesting;
using System.IO;

[TestClass]
public class PathUtilTest
{
    [TestMethod]
    public void ShouldConvertAbsolutePathToRelativePath()
    {
        Assert.AreEqual(@"file.txt", PathUtil.AbsoluteToRelative(new DirectoryInfo(@"C:\A\B\C"), new FileInfo(@"C:\A\B\C\file.txt")));
        Assert.AreEqual(@"D\file.txt", PathUtil.AbsoluteToRelative(new DirectoryInfo(@"C:\A\B\C"), new FileInfo(@"C:\A\B\C\D\file.txt")));
        Assert.AreEqual(@"..\..\file.txt", PathUtil.AbsoluteToRelative(new DirectoryInfo(@"C:\A\B\C"), new FileInfo(@"C:\A\file.txt")));
        Assert.AreEqual(@"..\..\D\file.txt", PathUtil.AbsoluteToRelative(new DirectoryInfo(@"C:\A\B\C"), new FileInfo(@"C:\A\D\file.txt")));

        Assert.AreEqual(@"file.txt", PathUtil.AbsoluteToRelative(new DirectoryInfo(@"C:\A\B\C\"), new FileInfo(@"C:\A\B\C\file.txt")));
        Assert.AreEqual(@"D\file.txt", PathUtil.AbsoluteToRelative(new DirectoryInfo(@"C:\A\B\C\"), new FileInfo(@"C:\A\B\C\D\file.txt")));
        Assert.AreEqual(@"..\..\file.txt", PathUtil.AbsoluteToRelative(new DirectoryInfo(@"C:\A\B\C\"), new FileInfo(@"C:\A\file.txt")));
        Assert.AreEqual(@"..\..\D\file.txt", PathUtil.AbsoluteToRelative(new DirectoryInfo(@"C:\A\B\C\"), new FileInfo(@"C:\A\D\file.txt")));
    }
}