在第二个依赖于第一个时执行异步任务

Question

I need some help regarding asynchronous programming. For simplicity, consider a list of integers 2, 1, 3, 3, 4, 5

I have two async tasks. One to get record and other to create record. I only want to create a record if the id property does not exist yet.

 public async Task<Record> GetRecord(int id)
 {
   var client = new HttpClient();
   // build request
   // var response = await client.SendAsync(request); // sending to 3rd party vendor
   // depending on response, return the Record object or null
 }

 public async Task<bool> CreateRecord(int id)
 {
   var client = new HttpClient();
   // build request
   // var response = await client.SendAsync(request); // sending to 3rd party vendor
   // depending on response, return true or false
 }

Main program

foreach(var id in IDs)
{
  var record = await GetRecord(id); ?? How can I wait here 
  if (record == null)
     await CreateRecord(id)
}

For some reason, this program is creating records with 2, 1, 3, 3, 4, 5. It should not be creating a duplicate record with id 3.

I also tried the following but it also creates duplicate records

 var record = Task.Run( async => await GetRecord(id)).Result

Any help would be greatly appreciated.

Answer 1

Here is a sample I tried showing what you have should work fine.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace TestSync
{
public class Record
{
    public int Id { get; set; }

    public Record(int id)
    {
        this.Id = id;
    }
}
public class TestClass
{
    private List<Record> SavedRecords = new List<Record>();
    private List<int> Ids = new List<int>{1, 2, 1, 3, 3, 4, 5, 3, 3, 4, 4, 2, 2};
    private Random randome = new Random(8383838);
    public async Task Process()
    {
        foreach (var id in Ids)
        {
            var record = await GetRecord(id);

            if (record == null)
                await CreateRecord(id);
        }
    }

    public async Task<Record> GetRecord(int id)
    {
        await Task.Delay(randome.Next(100, 200));// simulate delay for a HttpClient kind of call
        return SavedRecords.FirstOrDefault(i => i.Id == id);
    }
    public async Task<bool> CreateRecord(int id)
    {
        await Task.Delay(randome.Next(200, 500));// simulate delay for a HttpClient kind of call
        SavedRecords.Add(new Record(id));
        return true;
    }

    public void Print()
    {
        foreach (var record in SavedRecords)
            Console.WriteLine(record.Id);
    }
}
class Program
{

    static async Task Main(string[] args)
    {
        Console.WriteLine("Creating records!");

        var t = new TestClass();
        await t.Process();
        t.Print();
        Console.WriteLine("Done creating records!");
        Console.ReadKey();
    }
}

}