var fullPath = @"C:\folder1\folder2\file.txt";
var lastDirectory = Path.GetDirectoryName(fullPath).Split('\\').LastOrDefault();

DirectoryInfo执行剥离目录名的工作

string my_path = @"C:\Windows\System32";
DirectoryInfo dir_info = new DirectoryInfo(my_path);
string directory = dir_info.Name;  // System32

另一种方法是分割路径，使用c# 8.0中引入的Index Struct从路径中获取倒数第二个元素。

var path = @"C:\folder1\folder2\file.txt";
var folder = path.Split(@"\")[^2]; // 2nd element from the end

Console.WriteLine(folder); // folder2

这很难看，但避免了分配:

private static string GetFolderName(string path)
{
    var end = -1;
    for (var i = path.Length; --i >= 0;)
    {
        var ch = path[i];
        if (ch == System.IO.Path.DirectorySeparatorChar ||
            ch == System.IO.Path.AltDirectorySeparatorChar ||
            ch == System.IO.Path.VolumeSeparatorChar)
        {
            if (end > 0)
            {
                return path.Substring(i + 1, end - i - 1);
            }

            end = i;
        }
    }

    if (end > 0)
    {
        return path.Substring(0, end);
    }

    return path;
}

var fullPath = @"C:\folder1\folder2\file.txt";
var lastDirectory = Path.GetDirectoryName(fullPath).Split('\\').LastOrDefault();

string Folder = Directory.GetParent(path).Name;