using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = "ABCD";
            List<string> list = new List<string>();
            foreach (var i in Combo(s, 4))
            {
                list = list.Union(Arrange(i)).ToList();
            }
            list.ForEach(x => Console.WriteLine(x));
        }


        static IEnumerable<string> Arrange(string source)
        {
            for (int i = 0; i < source.Length; i++)
            {
                if (source.Length == 1)
                {
                    yield return source;
                }
                else
                {
                    foreach (var x in Arrange(source.Substring(0, i) + source.Substring(i + 1)))
                    {
                        yield return source[i] + x;
                    }
                }
            }
        }

        static IEnumerable<string> Combo(string source, int len)
        {
            int[] pos = new int[len];
            for (int i = 0; i < len; i++) pos[i] = i;
            while (pos[0] < source.Length - len)
            {
                string str = "";
                for (int i = 0; i < len; i++) str += source[pos[i]];
                for (int i = len - 1; i >= 0; i--)
                {
                    if (pos[i] < source.Length - len + i)
                    {
                        pos[i]++;
                        for (int j = i + 1; j <= len - 1; j++)
                        {
                            pos[j] = pos[i] + j - i;
                        }
                        break;
                    }
                    else
                    {
                        continue;
                    }
                }
                yield return str;
            }
            yield return source.Substring(source.Length - len);
        }
    }
}

------解决方案--------------------

另外，写为Linq查询表达式也是一样的。因为Linq也会使用yield迭代器来延迟计算，所以不用担心Linq的效率问题。使用Linq看起来逻辑上更优雅：
C# code
using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            排列组合("ABC", 0, 3).ToList().ForEach(x => { Console.WriteLine(x); });
            Console.ReadKey();
        }

        private static IEnumerable<string> 排列组合(string source, int fr, int len)
        {
            if (len == 1)
                return new List<string> { source.Substring(fr, 1) };
            else
                return from sub in 排列组合(source, fr + 1, len - 1)
                       from i in Enumerable.Range(0, sub.Length + 1)
                       let first = source.Substring(fr, 1)
                       select sub.Insert(i, first);
        }
    }
}