webAPi2 Invalid cast converting JSON to structure

Question

I am getting this error trying to call a web2.0 api call.

Message : "Unable to cast object of type 'Newtonsoft.Json.Linq.JObject' to type 'NG_API_DNET_FRX.Models.mproject'."

Here is the JSON that is being sent in from the webpage. The website uses Angular.

{
    "id": "3137",
    "clientId": "2",
    "Name": "MFAQ project1",
    "EstimatedStartDate": "07/01/2022",
    "EstimatedEndDate": "07/08/2022",
    "ActualStartDate": "07/15/2022",
    "ActualEndDate": "07/22/2022",
    "EstimatedBudget": "44444.0000",
    "ActualBudget": "55555.0000"
}

Here is the JSON representation of what is passed in. I got this by hovering on the routine parameter and copying from the watch....

Note the double squigly {{...}} surrounding the object. JSONLint complains about this but it seems to be consistent across all the calls and web api deals with it.
?? is this my problem ??

{{
  "id": 3137,
  "clientId": 2,
  "Name": "MFAQ project1",
  "EstimatedStartDate": "07/13/2022",
  "EstimatedEndDate": "6/8/2022",
  "ActualStartDate": "6/15/2022",
  "ActualEndDate": "6/22/2022",
  "EstimatedBudget": 44444,
  "ActualBudget": 55555
}}

Here is the target structure

public class mproject
{
    public int id;
    public int clientId;
    public string Name;
    public string EstimatedStartDate;
    public string EstimatedEndDate;
    public string ActualStartDate;
    public string ActualEndDate;
    public decimal EstimatedBudget;
    public decimal ActualBudget;
    public string sbProperties;
    public string projectType;

    public mprojectRev[] Revisions;
}


     [System.Web.Http.HttpPatch]
            [Route("{itemId_}")]
               public IHttpActionResult PatchItemById([FromUri] int itemId_, [FromBody] mproject webForm_  )
            {
                //if the parameter is of type mproject webform is null
   

//If i change the type to dynamic or object, and then try to //it, this is where i get the error
 
                //mproject webForm_;
                //try
                //{
                //    webForm_ = (mproject)webForm_1;
                //}
                //catch (Exception ex)
                //{
                //    return JSONStringResultExtension.JSONString(this, errorAsJSON(ex), HttpStatusCode.InternalServerError);
    
                //
    }
             }

There is no inner exception.

The value passed in does not include 3 properties defined on mproject. I do not believe that is the problem because I have had success in that scenario before. I am working w/ dates today... that is most likely where the problem is.

While I of course would love someone to solve my problem for me, what I really need is to get more information on the exception so I know what is causing the problem.

So the question is : Why is this cast throwing an exception

What would really help is learning where can I find more information about why this cast is throwing an exception. If someone has troubleshooting techniques, I would love those as well.

this property is missing: public mprojectRev[] Revisions; if it optional change it to public mprojectRev[]? Revisions; — Mark Homer, Jun 08 '22 at 15:36
*So the question is : **Why is this cast throwing an exception*** please [edit] your code to share a [mcve], specifically the cast that throws the exception. Probably you are using the non-generic `JsonConvert.DeserializeObject(string)` and casting to `NG_API_DNET_FRX.Models` which is the wrong thing to do as explained in [Unable to cast object of type Newtonsoft.Json.Linq.JObject even though I am trying to cast to an object with matching properties](https://stackoverflow.com/a/48130933). You must do `JsonConvert.DeserializeObject(string)` instead. — dbc, Jun 08 '22 at 15:49
I believe the double-braces appear because Visual Studio adds them for cosmetic purposes when displaying a `JObject`. (This constantly causes confusion here.) To see the "raw" json, in the Immediate Window do `Console.WriteLine(myJObject)`. — dbc, Jun 08 '22 at 15:49
@dbc I added the web api header in. If that is typed, then I webform_ = null. If I type that as object, i get the expected json, then get the error mentioned in the post when i try to cast. — greg, Jun 10 '22 at 14:21
@dbc agreed on the double quotes.... just didnt want to make that assumption and have it be the issue.... — greg, Jun 10 '22 at 14:22
@greg - so your real problem is that, when you change `[FromBody] mproject webForm_` to `[FromBody] dynamic webForm_`, you get an invalid cast exception doing `webForm_ = (mproject)webForm_1;`? If so, then this looks to be an exact duplicate of [Unable to cast object of type Newtonsoft.Json.Linq.JObject even though I am trying to cast to an object with matching properties](https://stackoverflow.com/a/48130933/3744182). And why change `mproject webForm_` to `dynamic webForm_` anyway? What is the purpose of not telling the framework the type to bind? — dbc, Jun 12 '22 at 18:05

Mark Homer · Accepted Answer · 2022-06-08T15:46:28.253

these three properties are missing from your json post:

public string sbProperties;
public string projectType;

public mprojectRev[] Revisions;

if they are optional change to:

public string? sbProperties ;
public string? projectType;

public mprojectRev[]? Revisions;

if they are not optional you need to pass those values in too

if you open dev tools with f12 in browser and go to Network, click the request and select the response tab it should show you the full response sent back from the api which would probably detail the issues

webAPi2 Invalid cast converting JSON to structure

1 Answers1