从列表创建字符串的另一种方法

有这样的函数，它从id列表中创建字符串我尝试在50个线程中运行它，30秒后它挂起app

public string genText(List<string> list)
{
    string text = @"{""text"":""";
    for (int i = 0; i < list.Count; i++)
    {
        if (i < list.Count - 1)
            text += "!" + " ";
        else
            text += "!" + @""", ";
    }
    text += @"""start"":[";
    for (int i = 0, lim = -2; i < list.Count; i++)
    {
        lim += 2;
        if (i < list.Count - 1)
            text += @"""" + lim + @""",";
        else
            text += @"""" + lim + @"""], ";
    }
    text += @"""end"":[";
    for (int i = 1, lim = -1; i < list.Count + 1; i++)
    {
        lim += 2;
        if (i < list.Count)
            text += @"""" + lim + @""",";
        else
            text += @"""" + lim + @"""], ";
    }
    text += @"""type"":[";
    for (int i = 0; i < list.Count; i++)
    {
        if (i < list.Count - 1)
            text += @"""USER"",";
        else
            text += @"""USER""], ";
    }
    text += @"""objectId"":[";
    for (int i = 0; i < list.Count; i++)
    {
        if (i < list.Count - 1)
            text += @"""" + list[i] + @""",";
        else
            text += @"""" + list[i] + @"""]}";
    }
    return text;
}

我什么 ­­­­­­­­­­­­­­ 年代最好的(替代品)的写作方式等功能 ­­­­­­­­­­­­­­­­­­­­­­­ n ?

预期结果:{"text":"! ! ! ! ! ! ! ! ! ! ! ! ! ! !", "start":["0","2","4","6","8","10","12","14","16","18","20","22","24","26","28"], "end":["1","3","5","7","9","11","13","15","17","19","21","23","25","27","29"], "type":["USER","USER","USER","USER","USER","USER","USER","USER","USER","USER","USER","USER","USER","USER","USER"], "objectId":["524231066205","363249609235","509321690322","551832845250","273337925148","553378679618","552270073142","256641407","545454406232","548096729194","555315805314","553271555117","573655339037","518779453704","486606264340"]}

public string genText(List<string> list)
{
    StringBuilder text = new StringBuilder();
    text.Append(@"{""text"":""");
    for (int i = 0; i < list.Count; i++)
    {
        if (i < list.Count - 1)
            text.Append("!" + " ");
    ...
    return text.ToString();
}