How to get relative path from absolute path

.NET Core 2.0 has Path.GetRelativePath, else, use this.

/// <summary>
/// Creates a relative path from one file or folder to another.
/// </summary>
/// <param name="fromPath">Contains the directory that defines the start of the relative path.</param>
/// <param name="toPath">Contains the path that defines the endpoint of the relative path.</param>
/// <returns>The relative path from the start directory to the end path or <c>toPath</c> if the paths are not related.</returns>
/// <exception cref="ArgumentNullException"></exception>
/// <exception cref="UriFormatException"></exception>
/// <exception cref="InvalidOperationException"></exception>
public static String MakeRelativePath(String fromPath, String toPath)
{
    if (String.IsNullOrEmpty(fromPath)) throw new ArgumentNullException("fromPath");
    if (String.IsNullOrEmpty(toPath))   throw new ArgumentNullException("toPath");

    Uri fromUri = new Uri(fromPath);
    Uri toUri = new Uri(toPath);

    if (fromUri.Scheme != toUri.Scheme) { return toPath; } // path can't be made relative.

    Uri relativeUri = fromUri.MakeRelativeUri(toUri);
    String relativePath = Uri.UnescapeDataString(relativeUri.ToString());

    if (toUri.Scheme.Equals("file", StringComparison.InvariantCultureIgnoreCase))
    {
        relativePath = relativePath.Replace(Path.AltDirectorySeparatorChar, Path.DirectorySeparatorChar);
    }

    return relativePath;
}

A bit late to the question, but I just needed this feature as well. I agree with DavidK that since there is a built-in API function that provides this, you should use it. Here's a managed wrapper for it:

public static string GetRelativePath(string fromPath, string toPath)
{
    int fromAttr = GetPathAttribute(fromPath);
    int toAttr = GetPathAttribute(toPath);

    StringBuilder path = new StringBuilder(260); // MAX_PATH
    if(PathRelativePathTo(
        path,
        fromPath,
        fromAttr,
        toPath,
        toAttr) == 0)
    {
        throw new ArgumentException("Paths must have a common prefix");
    }
    return path.ToString();
}

private static int GetPathAttribute(string path)
{
    DirectoryInfo di = new DirectoryInfo(path);
    if (di.Exists)
    {
        return FILE_ATTRIBUTE_DIRECTORY;
    }

    FileInfo fi = new FileInfo(path);
    if(fi.Exists)
    {
        return FILE_ATTRIBUTE_NORMAL;
    }

    throw new FileNotFoundException();
}

private const int FILE_ATTRIBUTE_DIRECTORY = 0x10;
private const int FILE_ATTRIBUTE_NORMAL = 0x80;

[DllImport("shlwapi.dll", SetLastError = true)]
private static extern int PathRelativePathTo(StringBuilder pszPath, 
    string pszFrom, int dwAttrFrom, string pszTo, int dwAttrTo);

There is a Win32 (C++) function in shlwapi.dll that does exactly what you want: PathRelativePathTo()

I'm not aware of any way to access this from .NET other than to P/Invoke it, though.

I have used this in the past.

/// <summary>
/// Creates a relative path from one file
/// or folder to another.
/// </summary>
/// <param name="fromDirectory">
/// Contains the directory that defines the
/// start of the relative path.
/// </param>
/// <param name="toPath">
/// Contains the path that defines the
/// endpoint of the relative path.
/// </param>
/// <returns>
/// The relative path from the start
/// directory to the end path.
/// </returns>
/// <exception cref="ArgumentNullException"></exception>
public static string MakeRelative(string fromDirectory, string toPath)
{
  if (fromDirectory == null)
    throw new ArgumentNullException("fromDirectory");

  if (toPath == null)
    throw new ArgumentNullException("toPath");

  bool isRooted = (Path.IsPathRooted(fromDirectory) && Path.IsPathRooted(toPath));

  if (isRooted)
  {
    bool isDifferentRoot = (string.Compare(Path.GetPathRoot(fromDirectory), Path.GetPathRoot(toPath), true) != 0);

    if (isDifferentRoot)
      return toPath;
  }

  List<string> relativePath = new List<string>();
  string[] fromDirectories = fromDirectory.Split(Path.DirectorySeparatorChar);

  string[] toDirectories = toPath.Split(Path.DirectorySeparatorChar);

  int length = Math.Min(fromDirectories.Length, toDirectories.Length);

  int lastCommonRoot = -1;

  // find common root
  for (int x = 0; x < length; x++)
  {
    if (string.Compare(fromDirectories[x], toDirectories[x], true) != 0)
      break;

    lastCommonRoot = x;
  }

  if (lastCommonRoot == -1)
    return toPath;

  // add relative folders in from path
  for (int x = lastCommonRoot + 1; x < fromDirectories.Length; x++)
  {
    if (fromDirectories[x].Length > 0)
      relativePath.Add("..");
  }

  // add to folders to path
  for (int x = lastCommonRoot + 1; x < toDirectories.Length; x++)
  {
    relativePath.Add(toDirectories[x]);
  }

  // create relative path
  string[] relativeParts = new string[relativePath.Count];
  relativePath.CopyTo(relativeParts, 0);

  string newPath = string.Join(Path.DirectorySeparatorChar.ToString(), relativeParts);

  return newPath;
}

As pointed above .NET Core 2.x has implementation of Path.GetRelativePath.

Code below is adapted from sources and works fine with .NET 4.7.1 Framework.

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

//Adapted from https://github.com/dotnet/corefx/blob/master/src/Common/src/CoreLib/System/IO/Path.cs#L697
// by Anton Krouglov

using System.Runtime.CompilerServices;
using System.Diagnostics;
using System.Text;
using Xunit;

namespace System.IO {
    // Provides methods for processing file system strings in a cross-platform manner.
    // Most of the methods don't do a complete parsing (such as examining a UNC hostname), 
    // but they will handle most string operations.
    public static class PathNetCore {

        /// <summary>
        /// Create a relative path from one path to another. Paths will be resolved before calculating the difference.
        /// Default path comparison for the active platform will be used (OrdinalIgnoreCase for Windows or Mac, Ordinal for Unix).
        /// </summary>
        /// <param name="relativeTo">The source path the output should be relative to. This path is always considered to be a directory.</param>
        /// <param name="path">The destination path.</param>
        /// <returns>The relative path or <paramref name="path"/> if the paths don't share the same root.</returns>
        /// <exception cref="ArgumentNullException">Thrown if <paramref name="relativeTo"/> or <paramref name="path"/> is <c>null</c> or an empty string.</exception>
        public static string GetRelativePath(string relativeTo, string path) {
            return GetRelativePath(relativeTo, path, StringComparison);
        }

        private static string GetRelativePath(string relativeTo, string path, StringComparison comparisonType) {
            if (string.IsNullOrEmpty(relativeTo)) throw new ArgumentNullException(nameof(relativeTo));
            if (string.IsNullOrEmpty(path)) throw new ArgumentNullException(nameof(path));
            Debug.Assert(comparisonType == StringComparison.Ordinal ||
                         comparisonType == StringComparison.OrdinalIgnoreCase);

            relativeTo = Path.GetFullPath(relativeTo);
            path = Path.GetFullPath(path);

            // Need to check if the roots are different- if they are we need to return the "to" path.
            if (!PathInternalNetCore.AreRootsEqual(relativeTo, path, comparisonType))
                return path;

            int commonLength = PathInternalNetCore.GetCommonPathLength(relativeTo, path,
                ignoreCase: comparisonType == StringComparison.OrdinalIgnoreCase);

            // If there is nothing in common they can't share the same root, return the "to" path as is.
            if (commonLength == 0)
                return path;

            // Trailing separators aren't significant for comparison
            int relativeToLength = relativeTo.Length;
            if (PathInternalNetCore.EndsInDirectorySeparator(relativeTo))
                relativeToLength--;

            bool pathEndsInSeparator = PathInternalNetCore.EndsInDirectorySeparator(path);
            int pathLength = path.Length;
            if (pathEndsInSeparator)
                pathLength--;

            // If we have effectively the same path, return "."
            if (relativeToLength == pathLength && commonLength >= relativeToLength) return ".";

            // We have the same root, we need to calculate the difference now using the
            // common Length and Segment count past the length.
            //
            // Some examples:
            //
            //  C:\Foo C:\Bar L3, S1 -> ..\Bar
            //  C:\Foo C:\Foo\Bar L6, S0 -> Bar
            //  C:\Foo\Bar C:\Bar\Bar L3, S2 -> ..\..\Bar\Bar
            //  C:\Foo\Foo C:\Foo\Bar L7, S1 -> ..\Bar

            StringBuilder
                sb = new StringBuilder(); //StringBuilderCache.Acquire(Math.Max(relativeTo.Length, path.Length));

            // Add parent segments for segments past the common on the "from" path
            if (commonLength < relativeToLength) {
                sb.Append("..");

                for (int i = commonLength + 1; i < relativeToLength; i++) {
                    if (PathInternalNetCore.IsDirectorySeparator(relativeTo[i])) {
                        sb.Append(DirectorySeparatorChar);
                        sb.Append("..");
                    }
                }
            }
            else if (PathInternalNetCore.IsDirectorySeparator(path[commonLength])) {
                // No parent segments and we need to eat the initial separator
                //  (C:\Foo C:\Foo\Bar case)
                commonLength++;
            }

            // Now add the rest of the "to" path, adding back the trailing separator
            int differenceLength = pathLength - commonLength;
            if (pathEndsInSeparator)
                differenceLength++;

            if (differenceLength > 0) {
                if (sb.Length > 0) {
                    sb.Append(DirectorySeparatorChar);
                }

                sb.Append(path, commonLength, differenceLength);
            }

            return sb.ToString(); //StringBuilderCache.GetStringAndRelease(sb);
        }

        // Public static readonly variant of the separators. The Path implementation itself is using
        // internal const variant of the separators for better performance.
        public static readonly char DirectorySeparatorChar = PathInternalNetCore.DirectorySeparatorChar;
        public static readonly char AltDirectorySeparatorChar = PathInternalNetCore.AltDirectorySeparatorChar;
        public static readonly char VolumeSeparatorChar = PathInternalNetCore.VolumeSeparatorChar;
        public static readonly char PathSeparator = PathInternalNetCore.PathSeparator;

        /// <summary>Returns a comparison that can be used to compare file and directory names for equality.</summary>
        internal static StringComparison StringComparison => StringComparison.OrdinalIgnoreCase;
    }

    /// <summary>Contains internal path helpers that are shared between many projects.</summary>
    internal static class PathInternalNetCore {
        internal const char DirectorySeparatorChar = '\\';
        internal const char AltDirectorySeparatorChar = '/';
        internal const char VolumeSeparatorChar = ':';
        internal const char PathSeparator = ';';

        internal const string ExtendedDevicePathPrefix = @"\\?\";
        internal const string UncPathPrefix = @"\\";
        internal const string UncDevicePrefixToInsert = @"?\UNC\";
        internal const string UncExtendedPathPrefix = @"\\?\UNC\";
        internal const string DevicePathPrefix = @"\\.\";

        //internal const int MaxShortPath = 260;

        // \\?\, \\.\, \??\
        internal const int DevicePrefixLength = 4;

        /// <summary>
        /// Returns true if the two paths have the same root
        /// </summary>
        internal static bool AreRootsEqual(string first, string second, StringComparison comparisonType) {
            int firstRootLength = GetRootLength(first);
            int secondRootLength = GetRootLength(second);

            return firstRootLength == secondRootLength
                   && string.Compare(
                       strA: first,
                       indexA: 0,
                       strB: second,
                       indexB: 0,
                       length: firstRootLength,
                       comparisonType: comparisonType) == 0;
        }

        /// <summary>
        /// Gets the length of the root of the path (drive, share, etc.).
        /// </summary>
        internal static int GetRootLength(string path) {
            int i = 0;
            int volumeSeparatorLength = 2; // Length to the colon "C:"
            int uncRootLength = 2; // Length to the start of the server name "\\"

            bool extendedSyntax = path.StartsWith(ExtendedDevicePathPrefix);
            bool extendedUncSyntax = path.StartsWith(UncExtendedPathPrefix);
            if (extendedSyntax) {
                // Shift the position we look for the root from to account for the extended prefix
                if (extendedUncSyntax) {
                    // "\\" -> "\\?\UNC\"
                    uncRootLength = UncExtendedPathPrefix.Length;
                }
                else {
                    // "C:" -> "\\?\C:"
                    volumeSeparatorLength += ExtendedDevicePathPrefix.Length;
                }
            }

            if ((!extendedSyntax || extendedUncSyntax) && path.Length > 0 && IsDirectorySeparator(path[0])) {
                // UNC or simple rooted path (e.g. "\foo", NOT "\\?\C:\foo")

                i = 1; //  Drive rooted (\foo) is one character
                if (extendedUncSyntax || (path.Length > 1 && IsDirectorySeparator(path[1]))) {
                    // UNC (\\?\UNC\ or \\), scan past the next two directory separators at most
                    // (e.g. to \\?\UNC\Server\Share or \\Server\Share\)
                    i = uncRootLength;
                    int n = 2; // Maximum separators to skip
                    while (i < path.Length && (!IsDirectorySeparator(path[i]) || --n > 0)) i++;
                }
            }
            else if (path.Length >= volumeSeparatorLength &&
                     path[volumeSeparatorLength - 1] == PathNetCore.VolumeSeparatorChar) {
                // Path is at least longer than where we expect a colon, and has a colon (\\?\A:, A:)
                // If the colon is followed by a directory separator, move past it
                i = volumeSeparatorLength;
                if (path.Length >= volumeSeparatorLength + 1 && IsDirectorySeparator(path[volumeSeparatorLength])) i++;
            }

            return i;
        }

        /// <summary>
        /// True if the given character is a directory separator.
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal static bool IsDirectorySeparator(char c) {
            return c == PathNetCore.DirectorySeparatorChar || c == PathNetCore.AltDirectorySeparatorChar;
        }

        /// <summary>
        /// Get the common path length from the start of the string.
        /// </summary>
        internal static int GetCommonPathLength(string first, string second, bool ignoreCase) {
            int commonChars = EqualStartingCharacterCount(first, second, ignoreCase: ignoreCase);

            // If nothing matches
            if (commonChars == 0)
                return commonChars;

            // Or we're a full string and equal length or match to a separator
            if (commonChars == first.Length
                && (commonChars == second.Length || IsDirectorySeparator(second[commonChars])))
                return commonChars;

            if (commonChars == second.Length && IsDirectorySeparator(first[commonChars]))
                return commonChars;

            // It's possible we matched somewhere in the middle of a segment e.g. C:\Foodie and C:\Foobar.
            while (commonChars > 0 && !IsDirectorySeparator(first[commonChars - 1]))
                commonChars--;

            return commonChars;
        }

        /// <summary>
        /// Gets the count of common characters from the left optionally ignoring case
        /// </summary>
        internal static unsafe int EqualStartingCharacterCount(string first, string second, bool ignoreCase) {
            if (string.IsNullOrEmpty(first) || string.IsNullOrEmpty(second)) return 0;

            int commonChars = 0;

            fixed (char* f = first)
            fixed (char* s = second) {
                char* l = f;
                char* r = s;
                char* leftEnd = l + first.Length;
                char* rightEnd = r + second.Length;

                while (l != leftEnd && r != rightEnd
                                    && (*l == *r || (ignoreCase &&
                                                     char.ToUpperInvariant((*l)) == char.ToUpperInvariant((*r))))) {
                    commonChars++;
                    l++;
                    r++;
                }
            }

            return commonChars;
        }

        /// <summary>
        /// Returns true if the path ends in a directory separator.
        /// </summary>
        internal static bool EndsInDirectorySeparator(string path)
            => path.Length > 0 && IsDirectorySeparator(path[path.Length - 1]);
    }

    /// <summary> Tests for PathNetCore.GetRelativePath </summary>
    public static class GetRelativePathTests {
        [Theory]
        [InlineData(@"C:\", @"C:\", @".")]
        [InlineData(@"C:\a", @"C:\a\", @".")]
        [InlineData(@"C:\A", @"C:\a\", @".")]
        [InlineData(@"C:\a\", @"C:\a", @".")]
        [InlineData(@"C:\", @"C:\b", @"b")]
        [InlineData(@"C:\a", @"C:\b", @"..\b")]
        [InlineData(@"C:\a", @"C:\b\", @"..\b\")]
        [InlineData(@"C:\a\b", @"C:\a", @"..")]
        [InlineData(@"C:\a\b", @"C:\a\", @"..")]
        [InlineData(@"C:\a\b\", @"C:\a", @"..")]
        [InlineData(@"C:\a\b\", @"C:\a\", @"..")]
        [InlineData(@"C:\a\b\c", @"C:\a\b", @"..")]
        [InlineData(@"C:\a\b\c", @"C:\a\b\", @"..")]
        [InlineData(@"C:\a\b\c", @"C:\a", @"..\..")]
        [InlineData(@"C:\a\b\c", @"C:\a\", @"..\..")]
        [InlineData(@"C:\a\b\c\", @"C:\a\b", @"..")]
        [InlineData(@"C:\a\b\c\", @"C:\a\b\", @"..")]
        [InlineData(@"C:\a\b\c\", @"C:\a", @"..\..")]
        [InlineData(@"C:\a\b\c\", @"C:\a\", @"..\..")]
        [InlineData(@"C:\a\", @"C:\b", @"..\b")]
        [InlineData(@"C:\a", @"C:\a\b", @"b")]
        [InlineData(@"C:\a", @"C:\A\b", @"b")]
        [InlineData(@"C:\a", @"C:\b\c", @"..\b\c")]
        [InlineData(@"C:\a\", @"C:\a\b", @"b")]
        [InlineData(@"C:\", @"D:\", @"D:\")]
        [InlineData(@"C:\", @"D:\b", @"D:\b")]
        [InlineData(@"C:\", @"D:\b\", @"D:\b\")]
        [InlineData(@"C:\a", @"D:\b", @"D:\b")]
        [InlineData(@"C:\a\", @"D:\b", @"D:\b")]
        [InlineData(@"C:\ab", @"C:\a", @"..\a")]
        [InlineData(@"C:\a", @"C:\ab", @"..\ab")]
        [InlineData(@"C:\", @"\\LOCALHOST\Share\b", @"\\LOCALHOST\Share\b")]
        [InlineData(@"\\LOCALHOST\Share\a", @"\\LOCALHOST\Share\b", @"..\b")]
        //[PlatformSpecific(TestPlatforms.Windows)]  // Tests Windows-specific paths
        public static void GetRelativePath_Windows(string relativeTo, string path, string expected) {
            string result = PathNetCore.GetRelativePath(relativeTo, path);
            Assert.Equal(expected, result);

            // Check that we get the equivalent path when the result is combined with the sources
            Assert.Equal(
                Path.GetFullPath(path).TrimEnd(Path.DirectorySeparatorChar),
                Path.GetFullPath(Path.Combine(Path.GetFullPath(relativeTo), result))
                    .TrimEnd(Path.DirectorySeparatorChar),
                ignoreCase: true,
                ignoreLineEndingDifferences: false,
                ignoreWhiteSpaceDifferences: false);
        }
    }
}

As Alex Brault points out, especially on Windows, the absolute path (with drive letter and all) is unambiguous and often better.

Shouldn't your OpenFileDialog use a regular tree-browser structure?

To get some nomenclature in place, the RefDir is the directory relative to which you want to specify the path; the AbsName is the absolute path name that you want to map; and the RelPath is the resulting relative path.

Take the first of these options that matches:

• If you have different drive letters, there is no relative path from RefDir to AbsName; you must use the AbsName.
• If the AbsName is in a sub-directory of RefDir or is a file within RefDir then simply remove the RefDir from the start of AbsName to create RelPath; optionally prepend "./" (or ".\" since you are on Windows).
• Find the longest common prefix of RefDir and AbsName (where D:\Abc\Def and D:\Abc\Default share D:\Abc as the longest common prefix; it has to be a mapping of name components, not a simple longest common substring); call it LCP. Remove LCP from AbsName and RefDir. For each path component left in (RefDir - LCP), prepend "..\" to (AbsName - LCP) to yield RelPath.

To illustrate the last rule (which is, of course, by far the most complex), start with:

RefDir = D:\Abc\Def\Ghi
AbsName = D:\Abc\Default\Karma\Crucible

Then

LCP = D:\Abc
(RefDir - LCP) = Def\Ghi
(Absname - LCP) = Default\Karma\Crucible
RelPath = ..\..\Default\Karma\Crucible

While I was typing, DavidK produced an answer which suggests that you are not the first to need this feature and that there is a standard function to do this job. Use it. But there's no harm in being able to think your way through from first principles, either.

Except that Unix systems do not support drive letters (so everything is always located under the same root directory, and the first bullet therefore is irrelevant), the same technique could be used on Unix.

It's a long way around, but System.Uri class has a method named MakeRelativeUri. Maybe you could use that. It's a shame really that System.IO.Path doesn't have this.

Use:

RelPath = AbsPath.Replace(ApplicationPath, ".")

If you're sure that your absolute path 2 is always relative to absolute path, just remove the first N characters from path2, where N is the length of path1.

I'd split both of your paths at the directory level. From there, find the point of divergence and work your way back to the assembly folder, prepending a '../' everytime you pass a directory.

Keep in mind however, that an absolute path works everywhere and is usually easier to read than a relative one. I personally wouldn't show an user a relative path unless it was absolutely necessary.

If you have a readonly text box, could you not not make it a label and set AutoEllipsis=true?

alternatively there are posts with code for generating the autoellipsis yourself: (this does it for a grid, you would need to pass i the width for the text box instead. It isn't quite right as it hacks off a bit more than is necessary, and I haven;t got around to finding where the calculation is incorrect. it would be easy enough to modify to remove the first part of the directory rather than the last if you desire.

Private Function AddEllipsisPath(ByVal text As String, ByVal colIndex As Integer, ByVal grid As DataGridView) As String
    'Get the size with the column's width 
    Dim colWidth As Integer = grid.Columns(colIndex).Width

    'Calculate the dimensions of the text with the current font
    Dim textSize As SizeF = MeasureString(text, grid.Font)

    Dim rawText As String = text
    Dim FileNameLen As Integer = text.Length - text.LastIndexOf("\")
    Dim ReplaceWith As String = "\..."

    Do While textSize.Width > colWidth
        ' Trim to make room for the ellipsis
        Dim LastFolder As Integer = rawText.LastIndexOf("\", rawText.Length - FileNameLen - 1)

        If LastFolder < 0 Then
            Exit Do
        End If

        rawText = rawText.Substring(0, LastFolder) + ReplaceWith + rawText.Substring(rawText.Length - FileNameLen)

        If ReplaceWith.Length > 0 Then
            FileNameLen += 4
            ReplaceWith = ""
        End If
        textSize = MeasureString(rawText, grid.Font)
    Loop

    Return rawText
End Function

Private Function MeasureString(ByVal text As String, ByVal fontInfo As Font) As SizeF
    Dim size As SizeF
    Dim emSize As Single = fontInfo.Size
    If emSize = 0 Then emSize = 12

    Dim stringFont As New Font(fontInfo.Name, emSize)

    Dim bmp As New Bitmap(1000, 100)
    Dim g As Graphics = Graphics.FromImage(bmp)

    size = g.MeasureString(text, stringFont)
    g.Dispose()
    Return size
End Function

The function that uses URI returned "almost" relative path. It included directory that directly contains the file which relative path I wanted to get.

Some time ago I wrote a simple function that returns relative path of folder or file, and even if it's on another drive, it includes the drive letter as well.

Please take a look:

    public static string GetRelativePath(string BasePath, string AbsolutePath)
    {
        char Separator = Path.DirectorySeparatorChar;
        if (string.IsNullOrWhiteSpace(BasePath)) BasePath = Directory.GetCurrentDirectory();
        var ReturnPath = "";
        var CommonPart = "";
        var BasePathFolders = BasePath.Split(Separator);
        var AbsolutePathFolders = AbsolutePath.Split(Separator);
        var i = 0;
        while (i < BasePathFolders.Length & i < AbsolutePathFolders.Length)
        {
            if (BasePathFolders[i].ToLower() == AbsolutePathFolders[i].ToLower())
            {
                CommonPart += BasePathFolders[i] + Separator;
            }
            else
            {
                break;
            }
            i += 1;
        }
        if (CommonPart.Length > 0)
        {
            var parents = BasePath.Substring(CommonPart.Length - 1).Split(Separator);
            foreach (var ParentDir in parents)
            {
                if (!string.IsNullOrEmpty(ParentDir))
                    ReturnPath += ".." + Separator;
            }
        }
        ReturnPath += AbsolutePath.Substring(CommonPart.Length);
        return ReturnPath;
    }

here's mine:

public static string RelativePathTo(this System.IO.DirectoryInfo @this, string to)
{
    var rgFrom = @this.FullName.Split(new[] { Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar }, StringSplitOptions.RemoveEmptyEntries);
    var rgTo = to.Split(new[] { Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar }, StringSplitOptions.RemoveEmptyEntries);
    var cSame = rgFrom.TakeWhile((p, i) => i < rgTo.Length && string.Equals(p, rgTo[i])).Count();

    return Path.Combine(
        Enumerable.Range(0, rgFrom.Length - cSame)
        .Select(_ => "..")
        .Concat(rgTo.Skip(cSame))
        .ToArray()
    );
}

Play with something like:

private String GetRelativePath(Int32 level, String directory, out String errorMessage) {
        if (level < 0 || level > 5) {
            errorMessage = "Find some more smart input data";
            return String.Empty;
        }
        // ==========================
        while (level != 0) {
            directory = Path.GetDirectoryName(directory);
            level -= 1;
        }
        // ==========================
        errorMessage = String.Empty;
        return directory;
    }

And test it

[Test]
    public void RelativeDirectoryPathTest() {
        var relativePath =
            GetRelativePath(3, AppDomain.CurrentDomain.BaseDirectory, out var errorMessage);
        Console.WriteLine(relativePath);
        if (String.IsNullOrEmpty(errorMessage) == false) {
            Console.WriteLine(errorMessage);
            Assert.Fail("Can not find relative path");
        }
    }

In ASP.NET Core 2, if you want the relative path to bin\Debug\netcoreapp2.2 you can use the following combination:

using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
public class RenderingService : IRenderingService
{

    private readonly IHostingEnvironment _hostingEnvironment;
    public RenderingService(IHostingEnvironment hostingEnvironment)
    {
    _hostingEnvironment = hostingEnvironment;
    }

    public string RelativeAssemblyDirectory()
    {
        var contentRootPath = _hostingEnvironment.ContentRootPath;
        string executingAssemblyDirectoryAbsolutePath = System.IO.Path.GetDirectoryName(System.Reflection.Assembly.GetExecutingAssembly().Location);
        string executingAssemblyDirectoryRelativePath = System.IO.Path.GetRelativePath(contentRootPath, executingAssemblyDirectoryAbsolutePath);
        return executingAssemblyDirectoryRelativePath;
    }
}

Adapted answer from duplicate: How to make an absolute path relative to a particular folder? and updated to match parameter sequence as the .Net standard implementation

This is a more concise implementation for .Net Framework that has thorough guidance in the doc comments and accounts for absolute paths that have relative folder links which is missing from many other solutions.

Tested against Anton Krouglov's test cases in .Net 6 all cases match System.IO.Path.GetRelativePath :)

public static partial class PathUtilities
{
    /// <summary>
    /// Rebases file with <paramref name="path"/> to the folder specified by <paramref name="relativeTo"/>.
    /// </summary>
    /// <param name="path">Full file path (absolute)</param>
    /// <param name="relativeTo">Full base directory path (absolute) the result should be relative to. This path is always considered to be a directory.</param>
    /// <returns>Relative path to file with respect to <paramref name="relativeTo"/></returns>
    /// <remarks>Paths are resolved by calling the <seealso cref="System.IO.Path.GetFullPath(string)"/> method before calculating the difference. This will resolve relative path fragments:
    /// <code>
    /// "c:\test\..\test2" => "c:\test2"
    /// </code>
    /// These path framents are expected to be created by concatenating a root folder with a relative path such as this:
    /// <code>
    /// var baseFolder = @"c:\test\";
    /// var virtualPath = @"..\test2";
    /// var fullPath = System.IO.Path.Combine(baseFolder, virtualPath);
    /// </code>
    /// The default file path for the current executing environment will be used for the base resolution for this operation, which may not be appropriate if the input paths are fully relative or relative to different
    /// respective base paths. For this reason we should attempt to resolve absolute input paths <i>before</i> passing through as arguments to this method.
    /// </remarks>
    static public string GetRelativePath(string relativeTo, string path)
    {
        String pathSep = "\\";
        String itemPath = Path.GetFullPath(path);
        String baseDirPath = Path.GetFullPath(relativeTo); // If folder contains upper folder references, they get resolved here. "c:\test\..\test2" => "c:\test2"
        bool isDirectory = path.EndsWith(pathSep);

        String[] p1 = Regex.Split(itemPath, "[\\\\/]").Where(x => x.Length != 0).ToArray();
        String[] p2 = Regex.Split(relativeTo, "[\\\\/]").Where(x => x.Length != 0).ToArray();
        int i = 0;

        for (; i < p1.Length && i < p2.Length; i++)
            if (String.Compare(p1[i], p2[i], true) != 0)    // Case insensitive match
                break;

        if (i == 0)     // Cannot make relative path, for example if resides on different drive
            return itemPath;

        String r = String.Join(pathSep, Enumerable.Repeat("..", p2.Length - i).Concat(p1.Skip(i).Take(p1.Length - i)));
        if (String.IsNullOrEmpty(r)) return ".";
        else if (isDirectory && p1.Length >= p2.Length) // only append on forward traversal, to match .Net Standard Implementation of System.IO.Path.GetRelativePath
            r += pathSep;

        return r;
    }
}

Usage of this method:

string itemPath = @"C:\Program Files\Dummy Folder\MyProgram\Data\datafile1.dat";
string baseDirectory = @"C:\Program Files\Dummy Folder\MyProgram";
string result = PathUtilities.GetRelativePath(baseDirectory, itemPath);
Console.WriteLine(result);

Results in:

Data\datafile1.dat

RE: But if the file is at the root directory or 1 directory below root, then display the full path.
In this case we could modify the method, or simply perform an additional check:

 string itemPath = @"C:\Program Files\Dummy Folder\datafile1.dat";
 string baseDirectory = @"C:\Program Files\Dummy Folder\MyProgram";
 string result = PathUtilities.GetRelativePath(baseDirectory, itemPath);
 Console.WriteLine("Before Check: '{0}'", result);
 if (result.StartsWith("..\\"))
     result = itemPath;
 Console.WriteLine("After Check: '{0}'", result);

Result:

 Before Check: '..\datafile1.dat'
 After Check: 'C:\Program Files\Dummy Folder\datafile1.dat'