Working with JSON files in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this JSON serialization code?

Consider the following C# code that serializes an object to JSON. What will be printed?

C Sharp (C#)

using System;
using System.Text.Json;

public class Person {
    public string Name { get; set; }
    public int Age { get; set; }
}

public class Program {
    public static void Main() {
        var person = new Person { Name = "Alice", Age = 30 };
        string json = JsonSerializer.Serialize(person);
        Console.WriteLine(json);
    }
}

A {"Name":Alice,"Age":30}

B {"name":"Alice","age":30}

C {"Name":"Alice","Age":30}

D {Name:"Alice",Age:30}

💡 Hint

Remember that property names are serialized with exact casing by default.

The JsonSerializer in C# serializes properties with their exact names and string values are quoted. So the output includes property names with uppercase first letters and string values in quotes.

❓ Predict Output

intermediate

What error does this JSON deserialization code produce?

What error will this code produce when trying to deserialize the JSON string?

C Sharp (C#)

using System;
using System.Text.Json;

public class Person {
    public string Name { get; set; }
    public int Age { get; set; }
}

public class Program {
    public static void Main() {
        string json = "{\"Name\":\"Bob\", \"Age\":\"thirty\"}";
        var person = JsonSerializer.Deserialize<Person>(json);
        Console.WriteLine(person.Name);
    }
}

A System.NullReferenceException

B System.Text.Json.JsonException

C System.FormatException

D No error, prints 'Bob'

💡 Hint

Check the type of the 'Age' property and the JSON value type.

The JSON has the 'Age' property as a string "thirty" but the C# class expects an int. This causes a JsonException during deserialization.

🔧 Debug

advanced

Why does this JSON serialization ignore a property?

Given this class and serialization code, why is the 'Password' property missing in the JSON output?

C Sharp (C#)

using System;
using System.Text.Json;
using System.Text.Json.Serialization;

public class User {
    public string Username { get; set; }
    [JsonIgnore]
    public string Password { get; set; }
}

public class Program {
    public static void Main() {
        var user = new User { Username = "user1", Password = "secret" };
        string json = JsonSerializer.Serialize(user);
        Console.WriteLine(json);
    }
}

A Password is private and cannot be serialized.

B The serializer only serializes string properties named Username.

C Password is null so it is not serialized.

D The [JsonIgnore] attribute tells the serializer to skip the Password property.

💡 Hint

Look at the attributes above the Password property.

The [JsonIgnore] attribute instructs the serializer to exclude that property from the JSON output.

📝 Syntax

advanced

Which option correctly deserializes JSON with a custom property name?

Given this JSON string and class, which code correctly deserializes the JSON?

{"full_name":"John Doe","Age":25}

Class:

public class Person {
    [JsonPropertyName("full_name")]
    public string Name { get; set; }
    public int Age { get; set; }
}

A var person = JsonSerializer.Deserialize<Person>(json);

B var person = JsonSerializer.Deserialize<Person>(json, new JsonSerializerOptions { PropertyNameCaseInsensitive = true });

C var person = JsonSerializer.Deserialize<Person>(json, new JsonSerializerOptions { PropertyNamingPolicy = JsonNamingPolicy.CamelCase });

D var person = JsonSerializer.Deserialize<Person>(json, new JsonSerializerOptions { PropertyNameCaseInsensitive = false });

💡 Hint

The attribute [JsonPropertyName] maps JSON property names to class properties.

The [JsonPropertyName("full_name")] attribute tells the deserializer to map the JSON property 'full_name' to the Name property. So default deserialization works.

🚀 Application

expert

How many items are in the dictionary after this JSON deserialization?

Given this JSON string and code, how many key-value pairs does the resulting dictionary contain?

{"a":1,"b":2,"c":3}

Code:

using System;
using System.Text.Json;
using System.Collections.Generic;

public class Program {
    public static void Main() {
        string json = "{\"a\":1,\"b\":2,\"c\":3}";
        var dict = JsonSerializer.Deserialize>(json);
        Console.WriteLine(dict.Count);
    }
}

A 3

B 0

C 1

D Throws an exception

💡 Hint

Count the number of properties in the JSON object.

The JSON object has three properties: 'a', 'b', and 'c'. Deserializing into a dictionary creates three entries.