.NET Core 2.0 Answer

.NET Core 2.0 has Path.GetRelativePath which can be used like so:

var relativePath = Path.GetRelativePath(    @"C:\Program Files\Dummy Folder\MyProgram",    @"C:\Program Files\Dummy Folder\MyProgram\Data\datafile1.dat");

In the above example, the relativePath variable is equal to Data\datafile1.dat.

Alternative .NET Answer

@Dave's solution does not work when the file paths do not end with a forward slash character (/) which can happen if the path is a directory path. My solution fixes that problem and also makes use of the Uri.UriSchemeFile constant instead of hard coding "FILE".

/// <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.</returns>/// <exception cref="ArgumentNullException"><paramref name="fromPath"/> or <paramref name="toPath"/> is <c>null</c>.</exception>/// <exception cref="UriFormatException"></exception>/// <exception cref="InvalidOperationException"></exception>public static string GetRelativePath(string fromPath, string toPath){    if (string.IsNullOrEmpty(fromPath))    {        throw new ArgumentNullException("fromPath");    }    if (string.IsNullOrEmpty(toPath))    {        throw new ArgumentNullException("toPath");    }    Uri fromUri = new Uri(AppendDirectorySeparatorChar(fromPath));    Uri toUri = new Uri(AppendDirectorySeparatorChar(toPath));    if (fromUri.Scheme != toUri.Scheme)    {        return toPath;    }    Uri relativeUri = fromUri.MakeRelativeUri(toUri);    string relativePath = Uri.UnescapeDataString(relativeUri.ToString());    if (string.Equals(toUri.Scheme, Uri.UriSchemeFile, StringComparison.OrdinalIgnoreCase))    {        relativePath = relativePath.Replace(Path.AltDirectorySeparatorChar, Path.DirectorySeparatorChar);    }    return relativePath;}private static string AppendDirectorySeparatorChar(string path){    // Append a slash only if the path is a directory and does not have a slash.    if (!Path.HasExtension(path) &&        !path.EndsWith(Path.DirectorySeparatorChar.ToString()))    {        return path + Path.DirectorySeparatorChar;    }    return path;}

Windows Interop Answer

There is a Windows API called PathRelativePathToA that can be used to find a relative path. Please note that the file or directory paths that you pass to the function must exist for it to work.

var relativePath = PathExtended.GetRelativePath(    @"C:\Program Files\Dummy Folder\MyProgram",    @"C:\Program Files\Dummy Folder\MyProgram\Data\datafile1.dat");public static class PathExtended{    private const int FILE_ATTRIBUTE_DIRECTORY = 0x10;    private const int FILE_ATTRIBUTE_NORMAL = 0x80;    private const int MaximumPath = 260;    public static string GetRelativePath(string fromPath, string toPath)    {        var fromAttribute = GetPathAttribute(fromPath);        var toAttribute = GetPathAttribute(toPath);        var stringBuilder = new StringBuilder(MaximumPath);        if (PathRelativePathTo(            stringBuilder,            fromPath,            fromAttribute,            toPath,            toAttribute) == 0)        {            throw new ArgumentException("Paths must have a common prefix.");        }        return stringBuilder.ToString();    }    private static int GetPathAttribute(string path)    {        var directory = new DirectoryInfo(path);        if (directory.Exists)        {            return FILE_ATTRIBUTE_DIRECTORY;        }        var file = new FileInfo(path);        if (file.Exists)        {            return FILE_ATTRIBUTE_NORMAL;        }        throw new FileNotFoundException("A file or directory with the specified path was not found.",            path);    }    [DllImport("shlwapi.dll", SetLastError = true)]    private static extern int PathRelativePathTo(        StringBuilder pszPath,        string pszFrom,        int dwAttrFrom,        string pszTo,        int dwAttrTo);}