web-api POST body object always null

Question

I\'m still learning web API, so pardon me if my question sounds stupid.

I have this in my StudentController:

public HttpResponseMessage          


        

Answer 1

In my case the problem was the DateTime object I was sending. I created a DateTime with "yyyy-MM-dd", and the DateTime that was required by the object I was mapping to needed "HH-mm-ss" aswell. So appending "00-00" solved the problem (the full item was null because of this).

Answer 2

If the any of values of the request's JSON object are not the same type as expected by the service then the [FromBody] argument will be null.

For example, if the age property in the json had a float value:

"age":18.0

but the API service expects it to be an int

"age":18

then student will be null. (No error messages will be sent in the response unless no null reference check).

Answer 3

Seems like there can be many different causes of this problem...

I found that adding an OnDeserialized callback to the model class caused the parameter to always be null. Exact reason unknown.

using System.Runtime.Serialization;

// Validate request
[OnDeserialized]  // TODO: Causes parameter to be null
public void DoAdditionalValidatation() {...}

Answer 4

Just one more thing to look at... my model was marked as [Serializable] and that was causing the failure.

Answer 5

I was also trying to use the [FromBody], however, I was trying to populate a string variable because the input will be changing and I just need to pass it along to a backend service but this was always null

Post([FromBody]string Input]) 

So I changed the method signature to use a dynamic class and then convert that to string

Post(dynamic DynamicClass)
{
   string Input = JsonConvert.SerializeObject(DynamicClass);

This works well.

Answer 6

Just to add my history to this thread. My model:

public class UserProfileResource
{
    public Guid Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Phone { get; set; }

    public UserProfileResource()
    {
    }
}

The above object couldn't be serialized in my API Controller and would always return null. The issue was with Id of type Guid: everytime I passed empty string as an Id (being naive that it will automatically be converted to Guid.Empty) from my frontend I received null object as [FromBody] paramether.

Solution was either to

• pass valid Guid value
• or change Guid to String