在控制台应用程序中将 JSON 反序列化为 C# 中的列表

Question

Error

Cannot deserialize the current JSON object (eg {"name":"value"}) into type 'System.Collections.Generic.List`1[ConsoleApp1.Menu]' because the type requires a JSON array (eg [1,2,3]) to deserialize correctly. To fix this error either change the JSON to a JSON array (eg [1,2,3]) or change the deserialized type so that it is a normal .NET type (eg not a primitive type like integer, not a collection type like an array or List) that can be deserialized from a JSON object. JsonObjectAttribute can also be added to the type to force it to deserialize from a JSON object. Path 'Test', line 3, position 9.

Question

Why do I get this error?

At the end I want to achieve to write every menu item in the console. For now I am stuck with this error. How do I achieve a result (Output) like this:

ID: 1 Name: Frosted Pumpkin Bread
ID: 2 Name: Ground to Go
...

The Console application

using System;
using System.Collections.Generic;
using System.Net;
using Newtonsoft.Json;


namespace ConsoleApp1
{
    public class MenuItem
    {
        public string ID { get; set; }
        public string Description { get; set; }
        public string Summary { get; set; }
        public string Name { get; set; }
        public string ImageFile { get; set; }
    }

    public class Menu
    {
        public List<MenuItem> Test { get; set; }
    }


    class Program
    {

        static void Main(string[] args)
        {
          var webclient = new WebClient();
          var json = webclient.DownloadString(@"C:\Users\devuser\source\repos\ConsoleApp1\ConsoleApp1\data.json");
          var model = JsonConvert.DeserializeObject<List<Menu>>(json);

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

My JSON-File looks like this

{

  "Test": [

    {
      "ID": "1",
      "Description": "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ut, tenetur natus doloremque laborum quos iste ipsum rerum obcaecati impedit odit illo dolorum ab tempora nihil dicta earum fugiat. Temporibus, voluptatibus. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ut, tenetur natus doloremque laborum quos iste ipsum rerum obcaecati impedit odit illo dolorum ab tempora nihil dicta earum fugiat. Temporibus, voluptatibus.",
      "Summary": "A seasonal delight we offer every autumn.  Pumpking bread with just a bit of spice, cream cheese frosting with just a hint of home.",
      "Name": "Frosted Pumpkin Bread",
      "ImageFile": "pumpkinbread.png"
    },
    {
      "ID": "2",
      "Description": "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ut, tenetur natus doloremque laborum quos iste ipsum rerum obcaecati impedit odit illo dolorum ab tempora nihil dicta earum fugiat. Temporibus, voluptatibus. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ut, tenetur natus doloremque laborum quos iste ipsum rerum obcaecati impedit odit illo dolorum ab tempora nihil dicta earum fugiat. Temporibus, voluptatibus.",
      "Summary": "Love our coffee? Take it with you so you never have to be without!",
      "Name": "Ground to Go",
      "ImageFile": "granola.png"
    },
    {
      "ID": "3",
      "Description": "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ut, tenetur natus doloremque laborum quos iste ipsum rerum obcaecati impedit odit illo dolorum ab tempora nihil dicta earum fugiat. Temporibus, voluptatibus. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ut, tenetur natus doloremque laborum quos iste ipsum rerum obcaecati impedit odit illo dolorum ab tempora nihil dicta earum fugiat. Temporibus, voluptatibus.",
      "Summary": "Do you prefer to grind your own coffee? No problem, we'll give you the beans.",
      "Name": "Bean there, done that!",
      "ImageFile": "beans.jpg"
    },
    {
      "ID": "4",
      "Description": "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ut, tenetur natus doloremque laborum quos iste ipsum rerum obcaecati impedit odit illo dolorum ab tempora nihil dicta earum fugiat. Temporibus, voluptatibus. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ut, tenetur natus doloremque laborum quos iste ipsum rerum obcaecati impedit odit illo dolorum ab tempora nihil dicta earum fugiat. Temporibus, voluptatibus.",
      "Summary": "They're just as round as donuts, but far more healthy! Freshly made every morning before sunrise.",
      "Name": "Fresh Bagels",
      "ImageFile": "bagel.png"
    },
    {
      "ID": "5",
      "Description": "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ut, tenetur natus doloremque laborum quos iste ipsum rerum obcaecati impedit odit illo dolorum ab tempora nihil dicta earum fugiat. Temporibus, voluptatibus. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ut, tenetur natus doloremque laborum quos iste ipsum rerum obcaecati impedit odit illo dolorum ab tempora nihil dicta earum fugiat. Temporibus, voluptatibus.",
      "Summary": "We've got strawberries, blueberries, apples, bananas - we could list them all, but we'd prefer you come take a look!",
      "Name": "Fresh Fruit",
      "ImageFile": "strawberries.png"
    }
  ]
}

Answer 1

You're trying to deserialize the json as List<Menu>

JsonConvert.DeserializeObject<List<Menu>>(json);

But the json does not contain a List of Menu, it only contains a single Menu object. You can see that the json starts with { }

{
    "Test": [...]
}

{ } is a object, [ ] is an Array. So to be able to deserialize into a List of Menu your json should look like this:

[
    {
        "Test": [...]
    }
]

so that you can have multiple Menu, like this:

[
    {
        "Test": [...]
    },
    {
        "Test": [...]
    },
    {
        "Test": [...]
    },
]

I hope I was helpful

Answer 2

THANX A LOT FOR YOUR ANSWERS!

Here is the working code!

using System;
using System.Collections.Generic;
using System.Net;
using Newtonsoft.Json;


namespace ConsoleApp1
{
    public class MenuItem
    {
        public string ID { get; set; }
        public string Description { get; set; }
        public string Summary { get; set; }
        public string Name { get; set; }
        public string ImageFile { get; set; }
    }

    public class Menu
    {
        public List<MenuItem> Test { get; set; }
    }


    class Program
    {

        static void Main(string[] args)
        {
          var webclient = new WebClient();
          var json = webclient.DownloadString(@"C:\Users\devuser\source\repos\ConsoleApp1\ConsoleApp1\data.json");
          var model = JsonConvert.DeserializeObject<Menu>(json);

            foreach (var item in model.Test)
            {
                Console.WriteLine("ID: "+item.ID+" "+"Name: "+item.Name);
            }
        }
    }
}
```

Answer 3

Change this line

var model = JsonConvert.DeserializeObject<List<Menu>>(json);

to

var model = JsonConvert.DeserializeObject<Menu>(json);

This is because given json represents an object Menu which contains a List<MenuItem> called Test.

Sample code.

DotNetFiddle Code Link

PS Sample code used a string initialisation instead of loading json from a file.