public static List<List<T>> FullCombination<T>(List<T> lstSource)
{
    var n = lstSource.Count;
    var max = 1 << n;//1乘以2的n次方
    var lstResult = new List<List<T>>();
    for (var i = 0; i < max; i++)
    {
        var lstTemp = new List<T>();
        for (var j = 0; j < n; j++)
        {
            //i除以2的j次方
            if ((i >> j & 1) > 0)
            {
                lstTemp.Add(lstSource[j]);
            }
        }
        lstResult.Add(lstTemp);
    }
    lstResult.RemoveAt(0);
    return lstResult;
}

List<string> lst = new List<string>() { "a", "b", "c" };
List<List<string>> ll = FullCombination(lst);

for (int i = 0; i < ll.Count; i++)
{
    for (int j = 0; j < ll[i].Count; j++)
    {
        Console.Write(ll[i][j]);
    }
    Console.WriteLine();
}