How do I use HttpClient (C#) to send form URL encoded data?

To use HttpClient in C# to send form URL-encoded data, you need to perform the following steps:

1. Create an instance of HttpClient.
2. Create an instance of HttpContent, specifically FormUrlEncodedContent to hold your form data.
3. Set up your form data as key-value pairs.
4. Send the form data using the PostAsync method of HttpClient.

Here is a step-by-step example:

using System;
using System.Net.Http;
using System.Collections.Generic;
using System.Threading.Tasks;

class Program
{
    static async Task Main(string[] args)
    {
        // Base address is optional but helps in managing relative URIs
        var baseAddress = new Uri("http://yourapi.com/");

        using (var httpClient = new HttpClient { BaseAddress = baseAddress })
        {
            // Form data is typically sent as key-value pairs
            var formData = new List<KeyValuePair<string, string>>
            {
                new KeyValuePair<string, string>("key1", "value1"),
                new KeyValuePair<string, string>("key2", "value2")
            };

            // Encodes the key-value pairs for the ContentType 'application/x-www-form-urlencoded'
            HttpContent content = new FormUrlEncodedContent(formData);

            try
            {
                // Send a POST request to the specified Uri as an asynchronous operation.
                HttpResponseMessage response = await httpClient.PostAsync("api/endpoint", content);

                // Ensure we get a successful response.
                response.EnsureSuccessStatusCode();

                // Read the response as a string.
                string result = await response.Content.ReadAsStringAsync();
                Console.WriteLine(result);
            }
            catch (HttpRequestException e)
            {
                Console.WriteLine("Error: " + e.Message);
            }
        }
    }
}

In this example:

• baseAddress is the base URI of the API or website you are sending requests to.
• httpClient is the instance of HttpClient that will be used to send the request.
• formData is a list of key-value pairs that represent the form data you want to send.
• FormUrlEncodedContent is used to encode the form data and set the Content-Type header to application/x-www-form-urlencoded.
• PostAsync sends a POST request to the specified endpoint with the encoded form data.
• EnsureSuccessStatusCode will throw an exception if the HTTP response status code does not indicate success.
• ReadAsStringAsync reads the response body as a string.

Remember, when using HttpClient, it's recommended to create a single instance and reuse it throughout the life of an application, instead of creating a new instance for every request. This helps to reduce the number of sockets in TIME_WAIT state and improve the performance of your application.

Additionally, always ensure to handle exceptions that may occur during the request, such as HttpRequestException, and possibly others depending on the context of your use case.