Querying WebAPI - webapi Cannot deserialize the current JSON array

Question

I have built a Web API using MVC and it works as expected.

I am now trying to query the API from a console application and I am hitting an issue. I understand why I am getting the issue but I dont understand how to fix it.

My Code from the console application:

static HttpClient client = new HttpClient();
        static  void Main(string[] args)
        {

            RunAsync().GetAwaiter().GetResult();

        }

        static async Task RunAsync()
        {

            client.BaseAddress = new Uri(URL);
            client.DefaultRequestHeaders.Accept.Clear();
            client.DefaultRequestHeaders.Accept.Add(
            new MediaTypeWithQualityHeaderValue("application/json"));

            List<TagDetail> tagDetail = new List<TagDetail>();

            tagDetail = await GetTagDetailAsync("api/tagdetail/?tagname=myTag&startdate=010120190000&enddate=020120190000");
            Console.WriteLine(tagDetail.value);
        }

            static async Task<TagDetail> GetTagDetailAsync(string path)
        {
            List<TagDetail> tagdetail = new List<TagDetail>();
            HttpResponseMessage response = await client.GetAsync(path);
            var test = response.StatusCode;
            var test2 = response.Headers;
            if (response.IsSuccessStatusCode)
            {
                tagdetail = await response.Content.ReadAsAsync<List<TagDetail>>(
            new List<MediaTypeFormatter>
            {
                new XmlMediaTypeFormatter(),
                new JsonMediaTypeFormatter()
            }); 
            }
            return tagdetail;
        }

The error I am getting is on the lines:

tagDetail = await GetTagDetailAsync("api/tagdetail/?tagname=99TOTMW&startdate=010120190000&enddate=020120190000");

And

return tagdetail;

The Web API returns the data in JSON format which looks like:

{  
   "tagname":"myTag",
   "value":"99.99",
   "description":"myDescription",
   "units":"£",
   "quality":"Good",
   "timestamp":"2019-08-01T17:32:30"
},
{  
   "tagname":"myTag",
   "value":"22.22",
   "description":"myDescription",
   "units":"£",
   "quality":"Good",
   "timestamp":"2019-08-01T17:33:30"
}

The TagDetail class is just declaration of each of the fields you see above.

The webapi provide the means of selecting a date range so I would get numerous TagDetails back as a List but it can also return just one (I can get this working by changing my code a bit). I need it to work for either one result or multiple.

could you change your code to this and see if your error goes away, static async Task<IEnumerable<TagDetail>> GetTagDetailAsync(string path). Your web API serves a single TagDetail object but you are passing back a List<TagDetail>. Handle the number of records returned on the client side. Also, what do you expect tagDetail.value to be? — Simon Wilson
– Simon Wilson, Commented Aug 5, 2019 at 17:09

Ryan · Accepted Answer · 2019-08-06 02:05:00Z

As the comment has explained that you need to return List<TagDetail> for your GetTagDetailAsync.Then you could use foreach to loop the result.This will work for one or multiple TagDetail

static async Task RunAsync()
    {

        //other logic
        List<TagDetail> tagDetail = new List<TagDetail>();

        tagDetail = await GetTagDetailAsync("api/tagdetail/?tagname=myTag&startdate=010120190000&enddate=020120190000");

        foreach(var item in tagDetail)
        {
            Console.WriteLine(item.value);
        }
    }

        static async Task<List<TagDetail>> GetTagDetailAsync(string path)
    {
        List<TagDetail> tagdetail = new List<TagDetail>();
        HttpResponseMessage response = await client.GetAsync(path);
        var test = response.StatusCode;
        var test2 = response.Headers;
        if (response.IsSuccessStatusCode)
        {
            tagdetail = await response.Content.ReadAsAsync<List<TagDetail>>(
        new List<MediaTypeFormatter>
        {
            new XmlMediaTypeFormatter(),
            new JsonMediaTypeFormatter()
        }); 
        }
        return tagdetail;
    }

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