POST body to parameter through input formatter

Question

I'm trying to write my own Input Formatter which will read the request body, split it by line and pass it into a string array parameter in a controller action.

---

This works (passing the entire body as a string):

Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvcCore(options =>
    {
        options.InputFormatters.Add(new MyInputFormatter());
    }
}

MyInputFormatter.cs

public override async Task<InputFormatterResult> ReadRequestBodyAsync(InputFormatterContext context)
{
    using (StreamReader reader = new StreamReader(context.HttpContext.Request.Body))
    {
        return InputFormatterResult.Success(await reader.ReadToEndAsync());
    }
}

MyController.cs

[HttpPost("/foo", Name = "Foo")]
public IActionResult Bar([FromBody] string foo)
{
    return Ok(foo);
}

---

This doesn't work (parameter foo is null):

MyInputFormatter.cs

public override async Task<InputFormatterResult> ReadRequestBodyAsync(InputFormatterContext context)
{
    List<string> input = new List<string>();

    using (StreamReader reader = new StreamReader(context.HttpContext.Request.Body))
    {
        while (!reader.EndOfStream)
        {
            string line = (await reader.ReadLineAsync()).Trim();
            input.Add(line);
        }
    }

    return InputFormatterResult.Success(input.ToArray());
}

MyController.cs

[HttpPost("/foo", Name = "Foo")]
public IActionResult Bar([FromBody] string[] foo)
{
    return Ok(string.Join(" ", foo));
}

The difference is that in the controller I'm now accepting an array of strings instead of a string and in the formatter I'm reading the input line by line and in the end returning it as an array.

---

What am I missing? :/

---

EDIT: How my formatter actually looks, more or less (if it makes any difference):

    public class MyInputFormatter : InputFormatter
    {
        public MyInputFormatter()
        {
            this.SupportedMediaTypes.Add(new MediaTypeHeaderValue(MimeType.URI_LIST)); // "text/uri-list"
        }

        public override bool CanRead(InputFormatterContext context)
        {
            if (context == null) throw new ArgumentNullException(nameof(context)); // breakpoint here not reached

            if (context.HttpContext.Request.ContentType == MimeType.URI_LIST)
                return true;

            return false;
        }

        protected override bool CanReadType(Type dataType)
        {
            return typeof(string[]).IsAssignableFrom(dataType); // breakpoint here not reached
        }

        public override async Task<InputFormatterResult> ReadRequestBodyAsync(InputFormatterContext context)
        {
            List<string> input = new List<string>(); // breakpoint here not reached

            using (StreamReader reader = new StreamReader(context.HttpContext.Request.Body))
            {
                while (!reader.EndOfStream)
                {
                    string line = (await reader.ReadLineAsync()).Trim();

                    if (string.IsNullOrEmpty(line))
                    {
                        continue;
                    }

                    if (!line.StartsWith("foo-"))
                    {
                        return InputFormatterResult.Failure();
                    }

                    input.Add(line.Substring("foo-".Length));
                }
            }

            return InputFormatterResult.Success(input.ToArray());
        }

Answer 1

I've created a test input formatter with your code in the request handler method and it works fine, this is how it looks like:

public class TestInputFormatter : IInputFormatter
{
    public bool CanRead(InputFormatterContext context) => true;

    public async Task<InputFormatterResult> ReadAsync(InputFormatterContext context)
    {
        List<string> input = new List<string>();

        using (StreamReader reader = new StreamReader(context.HttpContext.Request.Body))
        {
            while (!reader.EndOfStream)
            {
                string line = (await reader.ReadLineAsync()).Trim();
                input.Add(line);
            }
        }

        return InputFormatterResult.Success(input.ToArray());
    }
}

I see only one point which could be wrong in your code - the registration of your input formatter. Documentation says: Formatters are evaluated in the order you insert them. The first one takes precedence. Try to register it like that:

options.InputFormatters.Insert(0, new TestInputFormatter());

It works in my test project with exactly such registration. Because when you call options.InputFormatters.Add it will be added to the end of the input formatters collection and probably your request will be handled by some other input formatter which is located first in that collection.

Answer 2

I figured out what was the problem in the end. I had a custom ModelBinder which was interfering, capturing anything that isn't a string and an implementation of a custom interface (which is used for other post data). That's why it worked for strings and other input payloads (implementations of the interface), but not a string array. That binder was supposed to be used for query parameters (to be able to process custom types), but ended up triggering for this POST payload as well.