public static class ApiManager
{
    static Queue<ApiTask> apiTasks = new Queue<ApiTask>();
    static Thread thread;
    static AutoResetEvent wakeUp;

    public static void Init()    // <--1
    {
        wakeUp = new AutoResetEvent(false);
        thread = new Thread(runner);
        thread.Start();
    }

    public static void AddTask(ApiTask task) //<--5
    {
        apiTasks.Enqueue(task);
        wakeUp.Set();
    }

    static void runner()
    {
        wakeUp.Reset();
        while (true)  //<--2
        {
            wakeUp.WaitOne();      //<--3
            while (apiTasks.Count > 0)
            {
                var task = apiTasks.Dequeue(); //<--4
                task.Start();
            }
            wakeUp.Reset();
        }
    }
}

public abstract class ApiTask
{
    protected abstract void DoJob(); //<-- 1
    protected AutoResetEvent IsDone = new AutoResetEvent(false);
    protected Exception RegisteredException = null;

    internal void Run()
    {
    }

    public void Start()
    {
        try
        {
            DoJob();
        }
        catch (Exception ex)
        {
            this.RegisteredException = ex;
        }

        IsDone.Set();
    }
}

public abstract class ApiTask<TResult> : ApiTask where TResult : new()
{
    protected TResult Result = new TResult();

    public TResult GetResult()
    {
        ApiManager.AddTask(this);
        this.IsDone.WaitOne();

        if (RegisteredException != null) throw RegisteredException;
        return Result;
    }
}

public abstract class CreateDocumentTask:ApiTask<APIDocument>
{
    DateTime _date;

    protected int CreateDocumentTask(DateTime date)
    {
        _date = date;
    }

    protected override void DoJob()
    {
        Result = APICREATEDOCUMENTFUNCTION(_date);
    }
}