C# Linq操作List的問題，求教！謝謝~该怎么解决

C# Linq操作List的問題，求教！謝謝~
初学Linq，对linq操作List不太熟悉，现有以下三点困惑，望各位多多指教：
假设有一个用户类，包括了存款帳號，存款名，存款金額和一个DataTable，简单的代码如下：
static void Main(string[] args)
{
List<user> list = new List<user>();
list.Add(new user() { struserNo = "A1", struserName = "張三", deposit = 10000，dtUser = null});
list.Add(new user() { struserNo = "A1", struserName = "張三", deposit = 20000，dtUser = null});

list.Add(new user() { struserNo = "A2", struserName = "李四", deposit = 30000，dtUser = null});
list.Add(new user() { struserNo = "A2", struserName = "李四", deposit = 40000，dtUser = null});

list.Add(new user() { struserNo = "A3", struserName = "王五", deposit = 44000，dtUser = null});
}
class user
{
public string struserNo { get; set; }//存款帳號
public string struserName { get; set; }//存款名
public int deposit{ get; set; }//存款金額
public DataTable dtUser { get; set; }//一個user的DataTable
}
代码里list.add()了几个用户，如果现在我想做以下三个操作该怎么去写代码呢？
1：取出所有存款帳號(struserNo)为“A1”的存款金額(deposit)之和
2：取出存款金額(deposit)为“40000”并且存款帳號(struserNo)为“A2”的存款名(struserName)
3: 如何在每一次执行list。add()事件之后都为user类里的dtUser(DataTable类型)加上一行struserNo = "存款帳號", struserName = "存款名", deposit = “存款金額”？(即是dtUser = null要如何作修改才能达到第三点要求？)

------解决方案--------------------

C# code

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

namespace AAA
{
    class Program
    {
        static void Main(string[] args)
        {
            List<user> list = new List<user>();
            list.Add(new user() { struserNo = "A1", struserName = "張三", deposit = 10000, dtUser = null });
            list.Add(new user() { struserNo = "A1", struserName = "張三", deposit = 20000, dtUser = null });
            list.Add(new user() { struserNo = "A1", struserName = "張三1", deposit = 20000, dtUser = null });
            list.Add(new user() { struserNo = "A1", struserName = "張三2", deposit = 20000, dtUser = null });
            list.Add(new user() { struserNo = "A1", struserName = "張三3", deposit = 20000, dtUser = null });

            list.Add(new user() { struserNo = "A2", struserName = "李四", deposit = 30000, dtUser = null });
            list.Add(new user() { struserNo = "A2", struserName = "李四", deposit = 40000, dtUser = null });
            list.Add(new user() { struserNo = "A2", struserName = "李四1", deposit = 40000, dtUser = null });
            list.Add(new user() { struserNo = "A2", struserName = "李四2", deposit = 40000, dtUser = null });
            list.Add(new user() { struserNo = "A2", struserName = "李四3", deposit = 40000, dtUser = null });
            list.Add(new user() { struserNo = "A3", struserName = "王五", deposit = 44000, dtUser = null });


           
            user u = null;
            double total = 0;
            String struserName = "";
            for (int i = 0; i < list.Count(); i++)
            {
                u = list[i];
                //1：取出所有存款帳號(struserNo)为“A1”的存款金額(deposit)之和
                if (u.struserNo.Equals("A1"))
                {
                    total += u.deposit;
                }

                //2：取出存款金額(deposit)为“40000”并且存款帳號(struserNo)为“A2”的存款名(struserName)
                if (u.struserNo.Equals("A2") && (u.deposit == 40000)) 
                {

                    if (struserName.Equals(""))
                    {
                        struserName = u.struserName;
                    }
                    else {
                        struserName += " , " +  u.struserName;
                    }
                }

                //3，我也不会
            }

           

            Console.WriteLine(total);

            Console.WriteLine(struserName);

            Console.ReadLine();
        }
        class user
        {
            public string struserNo { get; set; }//存款帳號
            public string struserName { get; set; }//存款名
            public int deposit { get; set; }//存款金額
            public DataTable dtUser { get; set; }//一個user的DataTable
        }
    }
}

C# Linq操作List的問題，求教！謝謝~该怎么解决

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