Why can’t I deserialize JSON with List?

• 🚀 System.Text.Json and Newtonsoft.Json handle List<T> deserialization differently, affecting compatibility and performance.
• ⚠️ Incorrect JSON formatting, mismatched data types, and missing constructors commonly cause deserialization failures.
• 🔍 .NET JSON serialization requires parameterless constructors for successful deserialization with System.Text.Json.
• 🏗️ Using [JsonInclude], custom converters, or explicit deserialization logic can resolve common issues.
• 📝 Nested lists require strict adherence to JSON structure to avoid data loss during deserialization.

Understanding JSON Deserialization in .NET

JSON deserialization converts JSON-formatted text into a structured C# object. This process is crucial for handling API responses, configuration files, and data exchanges in modern applications. In .NET, JSON deserialization is primarily handled using two libraries:

1. System.Text.Json (Built-in)

The System.Text.Json library, introduced in .NET Core 3.0, is optimized for performance but comes with stricter conventions.

2. Newtonsoft.Json (Third-party)

Newtonsoft.Json (also known as Json.NET) has been a popular choice for .NET developers due to its flexibility, attribute support, and ease of handling complex JSON structures.

Despite their capabilities, developers often face challenges when deserializing JSON into C# objects, particularly when dealing with List<T> properties.

Common Reasons JSON Deserialization Fails with List<T>

Deserializing JSON into C# objects can fail for a variety of reasons. Below are some of the most common causes of deserialization failures involving lists.

1. Incorrect JSON Structure

JSON must be formatted correctly to align with the C# model definition. The most frequent mistake is using an object instead of an array for List<T>.

Incorrect JSON Example

{ "products": { "id": 1, "name": "Laptop" } }

Correct JSON Example

{ "products": [ { "id": 1, "name": "Laptop" } ] }

A List<T> requires square brackets ([]) around the array items. Without them, deserialization will fail or return null values.

2. Mismatched Data Types

Ensure the C# property types match their JSON equivalents:

3. Missing Parameterless Constructors

System.Text.Json requires a public parameterless constructor to deserialize objects. If your class lacks one, the deserialization will fail.

Incorrect Example

public class Product
{
    public int Id { get; }
    public string Name { get; }

    public Product(int id, string name)
    {
        Id = id;
        Name = name;
    }
}

Correct Example

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    
    // Public parameterless constructor required for System.Text.Json
    public Product() { }
}

4. Immutable Objects Without Setters

If a class defines read-only properties, deserialization won’t populate them unless explicitly included using [JsonInclude].

Solution with JsonInclude

public class Product
{
    public int Id { get; init; }
    public string Name { get; init; }

    [JsonConstructor]
    public Product(int id, string name)
    {
        Id = id;
        Name = name;
    }
}

Fixing Issues with List<T> Deserialization

To ensure smooth deserialization of List<T>, follow these best practices.

1. Verify Proper JSON Formatting

Ensure that JSON is correctly structured with arrays where List<T> is expected.

2. Use Parameterless Constructors

If using System.Text.Json, add a public parameterless constructor to models.

3. Apply Serialization Attributes

Use annotations such as:

• [JsonPropertyName("custom_name")] (for mapping JSON keys to C# properties)
• [JsonInclude] (for including private or read-only properties)

public class Product
{
    [JsonPropertyName("product_id")]
    public int Id { get; set; }

    [JsonPropertyName("product_name")]
    public string Name { get; set; }
}

4. Convert Mismatched Data Types

If a JSON field type doesn't match the expected C# type:

var list = JsonSerializer.Deserialize<List<Product>>(jsonData);

You may explicitly parse it in some cases.

Comparing Newtonsoft.Json vs. System.Text.Json

Newtonsoft.Json

• More tolerant of discrepancies between JSON and C# models.
• Easier handling of missing fields or type mismatches.
• Example usage:

  var inventory = JsonConvert.DeserializeObject<Inventory>(jsonData);
  

System.Text.Json

• Faster but stricter in deserialization.
• Case-sensitive by default.
• Example usage:

  var inventory = JsonSerializer.Deserialize<Inventory>(jsonData);
  

• Requires explicit options for case-insensitive matching or legacy support.

var options = new JsonSerializerOptions
{
    PropertyNameCaseInsensitive = true
};

Handling Nested Lists and Complex Objects

When lists are nested inside other lists, ensuring correct JSON structure is vital.

C# Model with Nested Lists

public class Category
{
    public string Name { get; set; }
    public List<Product> Products { get; set; }
}

public class Store
{
    public List<Category> Categories { get; set; }
}

Corresponding JSON

{
    "categories": [
        {
            "name": "Electronics",
            "products": [
                { "id": 1, "name": "Phone" },
                { "id": 2, "name": "Laptop" }
            ]
        }
    ]
}

Converting Between IEnumerable<T> and List<T>

APIs sometimes return IEnumerable<T>, but a List<T> is required for deserialization.

Conversion Example

List<Product> productList = productEnumerable.ToList();

Alternatively, modify the model:

public class Inventory
{
    public IEnumerable<Product> Products { get; set; }
}

Enhancing Serialization with JsonSerializerOptions

System.Text.Json provides serialization options that can adjust deserialization behavior.

var options = new JsonSerializerOptions
{
    PropertyNameCaseInsensitive = true, // Ignores case differences
    IncludeFields = true, // Includes fields in serialization
    AllowTrailingCommas = true // Avoid errors from extra commas
};

var inventory = JsonSerializer.Deserialize<Inventory>(jsonData, options);

Best Practices for Reliable Deserialization

1. Ensure Correct JSON Format

   • Use valid JSON structures that match the C# objects.

2. Use JsonInclude and JsonPropertyName Attributes

   • Helps map fields correctly even when naming conventions differ.

3. Test Serialization and Deserialization

• Validate with sample JSON inputs before integrating into production systems.

4. Centralize JSON Handling Code
   • Use dedicated serialization helpers to improve maintainability.

By applying these best practices, you can significantly reduce errors when working with .NET JSON serialization, ensuring smooth conversions between JSON data and C# objects.

Citations

• Microsoft. (n.d.). System.Text.Json namespace documentation. Microsoft Docs. Retrieved from Microsoft Docs
• Newtonsoft. (n.d.). Json.NET documentation. Newtonsoft. Retrieved from Newtonsoft