Frombody String Parameter Is Giving Null

FromBody string parameter is giving null

By declaring the jsonString parameter with [FromBody] you tell ASP.NET Core to use the input formatter to bind the provided JSON (or XML) to a model. So your test should work, if you provide a simple model class

public class MyModel
{
    public string Key {get; set;}
}

[Route("Edit/Test")]
[HttpPost]
public void Test(int id, [FromBody] MyModel model)
{
    ... model.Key....
}

and a sent JSON like

{
    key: "value"
}

Of course you can skip the model binding and retrieve the provided data directly by accessing HttpContext.Request in the controller. The HttpContext.Request.Body property gives you the content stream or you can access the form data via HttpContext.Request.Forms.

I personally prefer the model binding because of the type safety.

string value is Empty when using FromBody in asp.net web api

Like the doc says :

When a parameter has [FromBody], Web API uses the Content-Type header
to select a formatter.

Only XML and JSON content-types are supported by default. So you need to use application/xml, application/json or register a custom IInputFormatter.

Next, you need to send a content that match the selected content-type.
For json, if the parameter is int send a number. If it's a class, send a json object. If it's a string, send a json string. Etc.

int => 14
string => "azerty"
class => { "propName" : "value" }
Array => []
... => ...

In your case you should send application/json content-type and as content :

"Hello string"

And not just

Hello string

Aspnet core json input formatter implementation

Aspnet core xml input formatter implementation

Example: Creating a CSV Media Formatter

Why is my complex [FromBody] parameter null?

In my case there was a problem with the definition of the complex type, I had a parameter marked as string while it should have been string[] and so the JSON parsed did not match the model.

The important part though is how I found this out:

When debugging any API method there is the magic ModelState property.
This property gives you information about any failures that occur while binding the data received to the expected parameters.

e.g:

here we can see the parameter (uploaded), and the property within that parameter which failed to bind correctly.

Check the definition of that property and you'll probably find an error.

Post parameter is always null

Since you have only one parameter, you could try decorating it with the [FromBody] attribute, or change the method to accept a DTO with value as a property, as I suggested here: MVC4 RC WebApi parameter binding

UPDATE: The official ASP.NET site was updated today with an excellent explanation: https://learn.microsoft.com/en-us/aspnet/web-api/overview/advanced/sending-html-form-data-part-1

In a nutshell, when sending a single simple type in the body, send just the value prefixed with an equal sign (=), e.g. body:

=test

Null value when Pass values [FromBody] to post method by Postman plugin

You can't bind a single primitive string using json and FromBody, json will transmit an object and the controller will expect an complex object (model) in turn. If you want to only send a single string then use url encoding.

On your header set

Content-Type: application/x-www-form-urlencoded

The body of the POST request message body should be =saeed (based on your test value) and nothing else. For unknown/variable strings you have to URL encode the value so that way you do not accidentally escape with an input character.

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Alternate 1

Create a model and use that instead.

Message body value: {"name":"saeee"}

c#

public class CustomModel {
    public string Name {get;set;}
}

Controller Method

public HttpResponseMessage Post([FromBody]CustomModel model)

Alternate 2

Pass primitive strings to your post using the URI instead of the message body.

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