Why is my JSON string being passed into my webapi showing up as null?

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I'm writing code for a console app that sends an HttpPostRequest to a WebAPI that will later send emails and text messages to a group of people. This information is stored as a JSON string on the console app and passed into the WebAPI. When posting, I receive StatusCode 200 indicating a successful operation, but when I check the API, I see that the value passed in is "null". Can someone tell me why this is? I have included the code for the console app and WebAPI.

What I have tried:

WebAPI Controller:

C#

[ApiController]
[Route("[controller]")]
public class GeneratorStatusController : ControllerBase
{
    Emails emailList = new Emails();
    public GeneratorStatusController(IGeneratorService generatorService)
    {
        this.generatorService = generatorService;
    }

    static List<string> strings = new List<string>()
    {
        "value0", "value1", "value2"
    };
    private readonly IGeneratorService generatorService;

    [HttpGet]
    public List<string> GetString()
    {
        return strings;
    }

    [HttpPost]
    public List<string> PostString(string input)
    {
        Console.WriteLine(input);

        // Send Emails Placeholder
        strings.Add(input); // When adding it to "strings", the value is null
        return strings;
    }
}

Console App that sends a post request:

C#

static async Task SendPost(string machineName, List<string> emailAddresses, List<string> phoneNumbers)
{
    string url = "https://localhost:44312/GeneratorStatus/";
    HttpClient client = new HttpClient();

    string name = JsonConvert.SerializeObject(machineName);
    string emails = JsonConvert.SerializeObject(emailAddresses);
    string phones = JsonConvert.SerializeObject(phoneNumbers);

    var jsonMessage = JsonConvert.SerializeObject(new[] 
                     {JsonConvert.DeserializeObject(name),
                      JsonConvert.DeserializeObject(emails),
                      JsonConvert.DeserializeObject(phones) });

    Console.WriteLine(jsonMessage);

    var response = await client.PostAsync(url, new StringContent(jsonMessage, Encoding.UTF8, "application/json"));
    Console.WriteLine(response);
}

Posted 29-Sep-20 3:05am

stevenlam505

Updated 29-Sep-20 3:20am

Richard Deeming

v2

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Richard Deeming · Answer 1 · 2020-09-29T03:20:00

You need to add the [FromBody] attribute to your input parameter:

C#

[HttpPost]
public List<string> PostString([FromBody] string input)

Model Binding in ASP.NET Core | Microsoft Docs[^]

You should also remove the extra serialization and deserialization from your calling method. There's no point serializing to JSON simply to deserialize again on the next line.

However, since the method input is a string, you will need to double-encode your request body. Otherwise, the media formatter will decode the supplied JSON, and then refuse to bind the string array to a string parameter.

C#

static async Task SendPost(string machineName, List<string> emailAddresses, List<string> phoneNumbers)
{
    string url = "https://localhost:44312/GeneratorStatus/";
    HttpClient client = new HttpClient();

    var jsonMessage = JsonConvert.SerializeObject(new[] 
    {
        machineName,
        emailAddresses,
        phoneNumbers
    });

    Console.WriteLine(jsonMessage);
    
    // Serialize it again so that the server will treat it as a string:
    jsonMessage = JsonConvert.SerializeObject(jsonMessage);
    Console.WriteLine(jsonMessage);

    var response = await client.PostAsync(url, new StringContent(jsonMessage, Encoding.UTF8, "application/json"));
    Console.WriteLine(response);
}

As mentioned in the solution to your previous question[^], you should really be using a strongly-typed model instead of passing a JSON string around.