为什么同步任务在完成后不释放内存

Question

我需要将5.000.000条记录从数据库转换为JSON，但是在4.000条记录之后，我的内存不足。

我正在使用Task进行思考，当任务完成时，GC从内存中清除线程中的所有内容。

public class Program()
{
    public static void Main(string[] args)
    {
        Program p = new Program();
        p.ExportUsingTask();
    }

    public void ExportUsingTask()
    {
        List<int> ids = Program.LoadID(); // trying dont keep DBContext references, so GC can free memory
        GC.Collect(); // GC can clear 130MB of memory, DBContext have no references anymore
        GC.WaitForPendingFinalizers();

        foreach (int item in ids)
        {
            Task job = new Task(() => new Program().Process(item));
            job.RunSynchronously();
            Task.WaitAll(job);
            job.Dispose();
            job = null;
            GC.Collect(); // GC don't clear memory, uses more and more memory at each iteration, until OutOfMemoryException
            GC.WaitForPendingFinalizers();
        }
    }

    public static List<int> LoadID()
    {
        List<int> ids = new List<int>();
        using (Context db = new Context())
        {
            ids = db.Alfa.Where(a => a.date.Year == 2019).Select(a => a.id).ToList<int>(); // load 500.000 id from DB, use 130MB of memory
            // have some business logic here, but isn't the problem, memory is free after execution anyway
            db.Dispose();
        }

        return ids;
    }

    public void Process(int id)
    {
        Beta b = GetBetaFromAlfa(id); // Beta is JSON model that I need save to file
        string json = Newtonsoft.Json.JsonConvert.SerializeObject(b);
        b = null;

        using (StreamWriter sw = System.IO.File.AppendText(@"c:\MyFile.json"))
        {
            sw.Write(json);
            sw.Close();
            sw.Dispose();
        }

        GC.Collect(); // GC don't clear memory
        GC.WaitForPendingFinalizers();
    }

    public static Beta GetBetaFromAlfa(int idAlfa)
    {
        Alfa a = null; // Alfa is my model in DB
        Beta b = null; // Beta is JSON model that I need save to file

        using (Context db = new Context())
        {
            Alfa a = db.Alfa.Single(a => a.id == idAlfa); 
            b = ConvertAlfaToBeta(a);
            db.Dispose();
        }

        GC.Collect(); // GC don't clear memory
        GC.WaitForPendingFinalizers();

        return b;
    }

    public static Beta ConvertAlfaToBeta(Alfa alfa)
    {
        // business logic, something like:
        // beta.id = alfa.id;
        // beta.name = alfa.name;
        // only simple type association (int, string, decimal, datetime, etc)
    }
}

public class Alfa(){ ... }
public class Beta(){ ... }

在第一次尝试中，我做了一个循环，一个接一个地读取记录，当我得到100条记录时，我将所有JSON保存到文件中。但是无论如何，当我使用循环获得4000条记录时，我的内存不足：

    public void ExportUsingLoop()
    {
        List<int> ids = Program.LoadID(); // trying dont keep DBContext references, so GC can free memory
        GC.Collect(); // GC can clear 130MB of memory, DBContext have no references anymore
        GC.WaitForPendingFinalizers();

        int count = 0;
        StringBuilder content = new StringBuilder();
        foreach (int item in ids)
        {
            count++;
            Beta b = GetBetaFromAlfa(id); // Beta is JSON model that I need save to file
            string json = Newtonsoft.Json.JsonConvert.SerializeObject(b);
            content.AppendLine(json);

            b = null;
            json = null;

            if(count % 100 == 0)
            {
                using (StreamWriter sw = System.IO.File.AppendText(@"c:\MyFile.json"))
                {
                    sw.Write(content.ToString());
                    content.Clear(); // just for clarification
                    sw.Close();
                    sw.Dispose();
                }

                GC.Collect(); // GC don't clear memory, uses more and more memory at each iteration, until OutOfMemoryException
                GC.WaitForPendingFinalizers();
            }
        }
    }