How to Call a REST API in C# and .NET: HttpClient and JSON Deserialization

Overview #

A production .NET engineering guide for calling RESTful APIs in C# using HttpClient, IHttpClientFactory, System.Text.Json deserialization, and async/await task patterns.

The HttpClient Socket Exhaustion Trap in C# #

A classic mistake in C# is wrapping HttpClient in a using statement for each request. Even after disposal, underlying TCP sockets linger in TIME_WAIT state, leading to socket exhaustion under heavy load. In production .NET 8/9 applications, always use IHttpClientFactory or a static singleton HttpClient instance.

High-Performance JSON Serialization with System.Text.Json #

Using System.Text.Json.JsonSerializer.DeserializeAsync() streams JSON directly from the HTTP response network stream into strongly-typed C# record objects with zero unnecessary memory allocations.

Setting Bearer Authentication Headers #

Assign authentication tokens via client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue('Bearer', apiKey) to ensure all outgoing requests carry valid credentials.

Code Example: Invoking AI API in C# .NET 9 with HttpClient #

using System;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Text.Json;
using System.Threading.Tasks;

class Program
{
    private static readonly HttpClient client = new HttpClient();

    static async Task Main()
    {
        client.BaseAddress = new Uri("https://api.apihundred.com/v1/");
        client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", "your_api_key");

        var payload = new
        {
            model = "gpt-6-sol",
            messages = new[] { new { role = "user", content = "Explain async in C# in 2 sentences." } }
        };

        var json = JsonSerializer.Serialize(payload);
        var content = new StringContent(json, Encoding.UTF8, "application/json");

        var response = await client.PostAsync("chat/completions", content);
        response.EnsureSuccessStatusCode();

        var responseBody = await response.Content.ReadAsStringAsync();
        Console.WriteLine($"C# Response: {responseBody}");
    }
}

Frequently Asked Questions #

Q: Why should I use IHttpClientFactory in ASP.NET Core?

IHttpClientFactory manages the lifetime of underlying HttpMessageHandler instances to prevent socket exhaustion and automatically handle DNS changes.

Q: Can C# stream Server-Sent Events (SSE)?

Yes, using response.Content.ReadAsStreamAsync() with an IAsyncEnumerable allows real-time token streaming in C#.

Q: Is there an official OpenAI SDK for C# / .NET?

Yes, Microsoft and OpenAI maintain the official OpenAI .NET library, which can point directly to API100 by setting Endpoint.