ASP.NET Core Pitfalls – Posting a String

This is another post on my ASP.NET Core pitfalls series. It is actually related to this one ASP.NET Core Pitfalls – Null Models in Post Requests.

What happens if you try to submit a string containing JSON as a POST? I mean, you always submit strings, but this time you don't want for it to be parsed into some class. Like this:

[HttpPost]

public IActionResult PostString([FromBody] string json) { ... }

You may be surprised to find out that this fails with a HTTP 415 Unsupported Media Type.

Now, you might be tempted to add an [Consumes] attribute to it, such as:

[HttpPost]

[Consumes("application/json")]

public IActionResult PostString([FromBody] string json) { ... }

And this will actually work! The problem is, or could be, that it requires the sender to send the appropriate Content-Type header ("application/json").

Another, better, solution is to just read the string from the from the request stream:

[HttpPost]

public IActionResult PostString()

{

    using var reader = new StreamReader(HttpContext.Request.Body));

    var json = reader.ReadToEnd();

    ...

}

Of course, this could be made asynchronous, as StreamReader has a ReadToEndAsync method.

Yet another option is to add a custom input formatter, a class that implements IInputFormatter, one that just accepts a string, which needs to be registered globally:

builder.Services.AddControllersWithViews(static options =>

{

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

});

One example would be:

internal class StringInputFormatter : IInputFormatter

{

    public bool CanRead(InputFormatterContext context)

    {

        var canRead = context.HttpContext.Request.Method == HttpMethods.Post 

            && context.HttpContext.Request.ContentLength > 0;

        return canRead;

    }

    public async Task<InputFormatterResult> ReadAsync(InputFormatterContext context)

    {

        using var reader = new StreamReader(context.HttpContext.Request.Body);

        var content = await reader.ReadToEndAsync();

        return await InputFormatterResult.SuccessAsync(content);

    }

}

As you can see that there's a lot more involved than with the other two solutions.

Hope this comes useful!