(400) Bad Request when trying to post simple value to an API

Question

I have this LoansController for a web api

[Route("api/[controller]")]
[ApiController]
public class LoansController : ControllerBase
{
    // GET api/values
    [HttpGet]
    public ActionResult<IEnumerable<string>> Get()
    {
        return new string[] { "value1", "value2" };
    }

    // POST api/loans
    [HttpPost]
    public void Post([FromBody] string value)
    {

    }
}

In PowerShell I can call

Invoke-WebRequest http://localhost:1113/api/loans -Body $postParams -Method Get

and it works fine (I get value1 and value2)

But when I try

$postParams = "{'value':'123'}"
Invoke-WebRequest http://localhost:1113/api/loans -Body $postParams -Method Post # -ContentType 'Application/json'

I just keep getting

Invoke-WebRequest : The remote server returned an error: (400) Bad Request.

What am I doing wrong?

I tried adding -ContentType 'Application/json' but it made no difference

What am I missing?

I also tried Invoke-RestMethod but with the same results..

Next I removed [FromBody] from the value param but value now comes in as null

Answer 1

Reason

It just happens because your action method is expecting a plain string from HTTP request's Body:

[HttpPost]
public void Post([FromBody] string value)
{

}

Here a plain string is a sequence of characters which is quoted by "". In other words, to represent the string, you need include the quotes before and after these characters when sending request to this action method.

If you do want to send the json string {'value':'123'} to server, you should use the following payload :

POST http://localhost:1113/api/loans HTTP/1.1
Content-Type: application/json

"{'value':'123'}"

Note : We have to use doublequoted string ! Don't send string without the ""

How to fix

1. To send a plain string, simply use the following PowerShell scripts :

   $postParams = "{'value':'123'}"
   $postParams = '"'+$postParams +'"'
   Invoke-WebRequest http://localhost:1113/api/loans -Body $postParams  -Method Post  -ContentType 'application/json'
   

2. Or if you would like to send the payload with json, you could create a DTO to hold the value property:

   public class Dto{
       public string Value {get;set;}
   }
   

   and change your action method to be :

   [HttpPost]
   public void Post(Dto dto)
   {
       var value=dto.Value;
   }
   

   Finally, you can invoke the following PowerShell scripts to send request :

   $postParams = '{"value":"123"}'
   Invoke-WebRequest http://localhost:1113/api/loans -Body $postParams  -Method Post  -ContentType 'application/json'
   

These two approaches both work flawlessly for me.

Answer 2

Try to add headers

$url = 'http://localhost:1113/api/loans'
$head = @{'ContentType'='Application/json'}
Invoke-WebRequest -Uri $url -Body $postParams -Method Post -Headers $head