How do I get a value from the body of a web api POST?

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My ASP.NET Core 2.2 web api POST Calls are not getting anything from the body. Even when using a single field on the body, the value read in is null on POST. How do I resolve this?

What I have tried:

This is the current code in the API Controller:

[HttpPost("TestUser")]
 public async Task<IActionResult> TestUserAsync(string userName)
 {
     if (ModelState.IsValid)
     {
         return Ok(userName);
     }
     else
     {
         return BadRequest();
     }
 }

I have also tried this code with the [FromBody] in place (then the response from postman becomes "invalid input" and the method is not called.

I am calling it from postman:

"http://localhost:5502/LDH/User/TestUser"

Content Type is Application/Json and this is the raw string passed in as the body:

{"userName" : "testUser"}

Some of my research suggested that custom middleware logging could cause it by consuming the stream and not ressetting it- I use log4Net. I cannot find anything on how to reset the stream so am unable to rule that out.

The modelstate is valid, and not showing anything wrong

Posted 11-Nov-19 20:12pm

Ger Hayden

Updated 12-Nov-19 4:57am

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Richard Deeming · Answer 1 · 2019-11-12T04:58:00

Parameter Binding in ASP.NET Web API - ASP.NET 4.x | Microsoft Docs[^]:
By default, Web API uses the following rules to bind parameters:

If the parameter is a "simple" type, Web API tries to get the value from the URI. Simple types include the .NET primitive types (int, bool, double, and so forth), plus TimeSpan, DateTime, Guid, decimal, and string, plus any type with a type converter that can convert from a string. (More about type converters later.)
For complex types, Web API tries to read the value from the message body, using a media-type formatter.

Your parameter is a string, so model binding will try to get the value from the URL.

If you add [FromBody] to the parameter, it will use the media type formatter to read the parameter from the request body. For a string parameter with the application/json content type, it expects a single string enclosed in quotes:

POST http://localhost:5076/api/values HTTP/1.1
User-Agent: Fiddler
Host: localhost:5076
Content-Type: application/json
Content-Length: 7

"Alice"

Your body doesn't match that - you're passing in a whole object:

{"userName" : "testUser"}

Rick Strahl posted an approach to bind a string parameter directly to the raw request body back in 2013, which you might be able to adapt:
Accepting Raw Request Body Content with ASP.NET Web API - Rick Strahl's Web Log[^]

Otherwise, the simplest solution is probably to create a class to represent your parameter:

C#

public class TestUserParameters
{
    public string UserName { get; set; }
}
...
[HttpPost("TestUser")]
public async Task<IActionResult> TestUserAsync(TestUserParameters model) { ... }

Kornfeld Eliyahu Peter · Answer 2 · 2019-11-11T22:41:00

Solution 1

LMGTFY[^]
And read the first 5 links...

Posted 11-Nov-19 22:41pm

Kornfeld Eliyahu Peter

Comments

Ger Hayden 12-Nov-19 5:30am

Sorry, that is pretty much what I have been doing (that is how I learned that the stream may be getting hijacked by the logger). I had a quick look at your search string and 3 of the first 5 hits deal with multiple parameters. This is not a multiple parameter scenario, and common fixes like FromBody have been tried to no avail.

I will try block off log4Net and see if that resolves the issue.