I am new to C# and trying to implement Async task for multiple API calls, but facing a problem of how to return the Task response.

This code below it works perfect for multiple API calls:

public class ItemResponseJsonModel
{
    public string ID { get; set; }
    public string ResponseJSON { get; set; }
}

var listIDs = new List<string>
{
    "ID1",
    "ID2",
    "ID3"
};

var postTasks = listIDs.Select(async p => new ItemResponseJsonModel { ID = p, ResponseJSON = await APIGetItemsByID(p) });
var posts = await System.Threading.Tasks.Task.WhenAll(postTasks);
foreach (var postContent in posts)
{
    listResults.Add(postContent);
}

Task<string> APIGetItemsByID(string ID)
{
    int page = 1;
    string? API_Key = "The-API-Key"
    Task<string> respJSON;

    var client = new HttpClient();
    string baseURL = $"https://API_URL/api/Items?pageSize=1000&Page={page}&id=";
    client.DefaultRequestHeaders.Add("Authorization", "Basic " + API_Key);
    respJSON = client.GetStringAsync(baseURL + ID);
    return respJSON;
}

Now I am running into the situation is that each ID could have multiple result pages. That last page is always return "[]" meaning no data, so I implemented the Do/While loop below to get all responses for each ID. This code below works but it is slow. Please let me if it can be approved.

async Task<string> APIGetItemsByID(string ID)
{
    int page = 1;
    string? API_Key = "The-API-Key"
    string respJSON = string.Empty;
    string respContent;

    do
    {
        var client = new HttpClient();
        string baseURL = $"https://API_URL/api/Items?pageSize=1000&Page={page}&id=";
        client.DefaultRequestHeaders.Add("Authorization", "Basic " + API_Key);        
        respContent = client.GetStringAsync(baseURL + ID);
        if (respContent != "[]") { 
            respJSON += respContent;
        } 
        page++;
    } while (respContent != "[]" && page < 10);
    return respJSON;
}

Any help is appreciated.

I am new to C# and trying to implement Async task for multiple API calls, but facing a problem of how to return the Task response.

This code below it works perfect for multiple API calls:

public class ItemResponseJsonModel
{
    public string ID { get; set; }
    public string ResponseJSON { get; set; }
}

var listIDs = new List<string>
{
    "ID1",
    "ID2",
    "ID3"
};

var postTasks = listIDs.Select(async p => new ItemResponseJsonModel { ID = p, ResponseJSON = await APIGetItemsByID(p) });
var posts = await System.Threading.Tasks.Task.WhenAll(postTasks);
foreach (var postContent in posts)
{
    listResults.Add(postContent);
}

Task<string> APIGetItemsByID(string ID)
{
    int page = 1;
    string? API_Key = "The-API-Key"
    Task<string> respJSON;

    var client = new HttpClient();
    string baseURL = $"https://API_URL/api/Items?pageSize=1000&Page={page}&id=";
    client.DefaultRequestHeaders.Add("Authorization", "Basic " + API_Key);
    respJSON = client.GetStringAsync(baseURL + ID);
    return respJSON;
}

Now I am running into the situation is that each ID could have multiple result pages. That last page is always return "[]" meaning no data, so I implemented the Do/While loop below to get all responses for each ID. This code below works but it is slow. Please let me if it can be approved.

async Task<string> APIGetItemsByID(string ID)
{
    int page = 1;
    string? API_Key = "The-API-Key"
    string respJSON = string.Empty;
    string respContent;

    do
    {
        var client = new HttpClient();
        string baseURL = $"https://API_URL/api/Items?pageSize=1000&Page={page}&id=";
        client.DefaultRequestHeaders.Add("Authorization", "Basic " + API_Key);        
        respContent = client.GetStringAsync(baseURL + ID);
        if (respContent != "[]") { 
            respJSON += respContent;
        } 
        page++;
    } while (respContent != "[]" && page < 10);
    return respJSON;
}

Any help is appreciated.

• c#
• asp

• Have you heard of the await keyword learn.microsoft/en-us/dotnet/csharp/language-reference/… You need to await the task result var respJSON = await client.GetStringAsync(baseURL + ID); – Charlieface Commented 17 hours ago
• I tried await respJSON = client.GetStringAsync(baseURL + QuoteID); and get an error "name space await could not found ...". Any suggestion? – Milacay Commented 17 hours ago
• 1 That's not what I wrote..... await goes on the function call, not on the variable. Honestly I think you should read up on how Task and await work, and take a good look at the syntax in the examples. You are also missing async at the top async Task<string> APIGetItemsByID(string ID) and you should declare respJSON as a string not a Task<string> – Charlieface Commented 17 hours ago
• Based on your suggestion. I changed to async at the top async Task<string> APIGetItemsByID(string ID), declare respJSON as a string, then var respContent = await client.GetStringAsync(baseURL + QuoteID);, then respJSON += respContent;. It works, but it is slow. Thanks for your suggestion. – Milacay Commented 16 hours ago
• @Milacay, whoever reads your ResponseJSON from ItemResponseJsonModel will have an invalid JSON string to work with. – SoftwareDveloper Commented 15 hours ago

1 Answer 1

If you want to implement paging in your API calls, your current approach is generally fine, but there are several improvements you can make.

First, you're creating a new HttpClient instance inside your APIGetItemsByID method, which is being called within a loop. This is not recommended. Each time you create a new HttpClient, it opens a new socket connection under the hood. Repeatedly creating instances in a loop can quickly exhaust system resources. Moreover, establishing new TCP connections repeatedly is slower than reusing an existing one.

A better approach is to create and reuse a single HttpClient instance, especially within loops or for high-frequency operations. In my example, I instantiate the HttpClient outside the loop and passing it to your APIGetItemsByID method.

You're also setting DefaultRequestHeaders on your HttpClient. Headers set here apply to all requests, so there's no need to add them to individual request messages. Keep in mind, though, that you should avoid modifying DefaultRequestHeaders if there are pending requests, as this can lead to unpredictable behavior.

In your do-while loop, you're checking whether the response is an empty array. That works, but for better readability and clarity, you might consider explicitly exiting the loop when an empty response is received.

Lastly, as mentioned in the comments, you're concatenating response data that comes in the form of arrays. If you simply append JSON arrays as strings, you may end up with an invalid JSON structure like "[...][...]". Instead, you should parse the JSON arrays and merge the array contents programmatically. This ensures you're working with valid JSON and makes further processing much easier.

Here are my improvements:

using System;
using System.Diagnostics;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using System.Net.Http;

public class ItemResponseJsonModel
{
    public string ID { get; set; }
    public string ResponseJSON { get; set; }
}

string baseUrl = "https://API_URL/api/Items";
string API_Key = "The-API-Key";
var client = new HttpClient();
client.DefaultRequestHeaders.Add("Authorization", "Basic " + API_Key);

var listIDs = new List<string>
{
    "ID1",
    "ID2",
    "ID3"
};

var postTasks = listIDs.Select(async p => new ItemResponseJsonModel { ID = p, ResponseJSON = await APIGetItemsByID(client, baseUrl, p) });
var posts = await System.Threading.Tasks.Task.WhenAll(postTasks);
foreach (var postContent in posts)
{
    listResults.Add(postContent);
}

async Task<string> APIGetItemsByID(client HttpClient, string baseUrl, string ID)
{
    int page = 1;
    string respJSON = string.Empty;

    do
    {
        string url = $"{baseUrl}?pageSize=1000&Page={page}&id={ID}";
        string responseContent = await client.GetStringAsync(url);
        
        if (responseContent == "[]") { 
            break;
        }

        respJSON += responseContent;
        page++;
    } 
    while (page < 10);

    return respJSON;
}