ASP.NET FormUrlEncode with child List KeyValuePair

Question

I'm calling an API using HTTP POST with a form URL encoded body

The controller I'm calling looks like this:

public async Task<IHttpActionResult> DoSomethingAsync(ABCRequest widget)

[DataContract]
public class ABCRequest
{
  [DataMember(Name="name")]
  public string Name { get; set; }

  [DataMember(Name="number")]
  public string Number { get; set; }
}

I can happily call the API using the following method

var formData = new List<KeyValuePair<string, string>>
{
    new KeyValuePair<string, string>("Name", "John"),
    new KeyValuePair<string, string>("Number", "123")
};

HttpContent content = new FormUrlEncodedContent(formData);

Another API endpoint uses

public async Task<IHttpActionResult> DoSomething2Async(ABCRequest2 widget)
    
    [DataContract]
    public class ABCRequest2
    {
      [DataMember(Name="name")]
      public string Name { get; set; }

      [DataMember(Name="number")]
      public string Number { get; set; }

      [DataMember(Name="metadata")]
      public List<KeyValuePair<string, string>> Metadata { get; set; }
    }

How do I form URL encode the child List?

Answer 1

How do I form URL encode the child List?

According ot your scenario and description along with the share code snippet, I have tried to simulate your scenario and I was able to send send child metadata list with URL encoded value.

In order to form URL encoded child list of KeyValuePair<string, string> in your ABCRequest2 class, you need to flatten the structure into key-value pairs that can be properly encoded.

First of all, you need to convert the child list into individual key-value pairs where each pair has a unique key representing its position or specific identifier.

After constructing the key-value pairs, have to use FormUrlEncodedContent to encode the flat list.

Let's have a look in practice, how we could achieve that:

From a key value pairs encoded content:

var formData = new List<KeyValuePair<string, string>>
{
    new KeyValuePair<string, string>("name", "John"),
    new KeyValuePair<string, string>("number", "123")
};

var metadataList = new List<KeyValuePair<string, string>>
{
    new KeyValuePair<string, string>("key1", "value1"),
    new KeyValuePair<string, string>("key2", "value2")
};

for (int i = 0; i < metadataList.Count; i++)
{
    formData.Add(new KeyValuePair<string, string>($"metadata[{i}].Key", metadataList[i].Key));
    formData.Add(new KeyValuePair<string, string>($"metadata[{i}].Value", metadataList[i].Value));
}

HttpContent content = new FormUrlEncodedContent(formData);

using (var client = new HttpClient())
{
    var response = await client.PostAsync("https://localhost:7246/api/Receiving/ReceiveData", content);
    if (response.IsSuccessStatusCode)
    {
        var responseData = await response.Content.ReadAsStringAsync();
        return Ok(responseData);
    }
    return BadRequest("Failed to send data");
}

Receiving Controller:

[Route("api/[controller]")]
[ApiController]
public class ReceivingController : ControllerBase
{
    [HttpPost]
    [Route("ReceiveData")]
    public async Task<IActionResult> ReceiveData([FromForm] ABCRequest2 widget)
    {
       
        return Ok(widget);
    }
}

Output:

Note: Please refer to this official document for additionl reference and sample