求一个字符串没有重复字符的最大子串长度

/// <summary>
/// 获取一个字符串的没有重复字符的最大子串长度，比如abcabcbb所求值为abc、bca、cab即为3
/// </summary>
/// <param name="args"></param>
static void Main(string[] args)
{
    string str = "abcabcbb";      
    int initlength = str.Length;         
    //长度位移法获取子串
    Dictionary<int, List<string>> strlist = new Dictionary<int, List<string>>();
    for (var i = 0; i < initlength-1; i++)
    {
        List<string> valuelist = new List<string>();
        var valuelength = initlength - 1 - i;
        for (var j = 0; j <= initlength - valuelength; j++)
        {
            var value = str.Substring(j, valuelength);  //(位移，长度)
            valuelist.Add(value);                  
        }
        strlist.Add(valuelength, valuelist);  
    }
    //单元素去重校验
    foreach (var item in strlist.OrderByDescending(p=>p.Key))
    {
        foreach (var itemchild in item.Value)
        {
            bool isRepeat = false;
            for (var i = 0; i < itemchild.ToCharArray().Length; i++)
            {
                for (var j = 0; j < itemchild.ToCharArray().Length; j++)
                {
                    if (i!=j && itemchild.ToCharArray()[i] == itemchild.ToCharArray()[j])
                    {
                        isRepeat = true;
                        break;
                    }
                }
                if (isRepeat)
                {
                    break;
                }
            }
            if (!isRepeat)
            {
                Console.WriteLine($"结果为:{item.Key}");
                Console.ReadLine();
            }
        }                         
    }
}