Lowercase JSON Attributes in C# API Responses

One standard approach is using the JsonPropertyName attribute, which allows you to specify the output for each field on an individual basis:

For example, Using JsonPropertyName:

using System.Text.Json.Serialization;

namespace MyCode
{
    public class Response
    {
        [JsonPropertyName("orderId")]
        public int OrderID { get; set; }

        [JsonPropertyName("userName")]
        public string UserName { get; set; }
    }
}

In this example, we are manually defining the JSON property names using JsonPropertyName, which is useful when you need specific control over each property.

However, if you want a more scalable solution to apply a consistent naming convention across all properties in your response, you can use a global naming policy instead.

Note: The JsonPropertyName attribute takes priority over the global naming policy. If you plan to use a global rule, make sure to remove these individual attributes from your model classes.

Create a Custom JsonNamingPolicy

using System.Text.Json;

namespace MyCode
{
    public class UppercaseNamingPolicy : JsonNamingPolicy
    {
        public override string ConvertName(string name)
        {
            return name.ToUpper();  // This converts the property name to uppercase
        }
    }
}

In this example, the custom UppercaseNamingPolicy class overrides the ConvertName method to return the property names in uppercase.

Apply the Naming Policy Globally

builder.Services.AddControllers()
    .AddJsonOptions(options =>
    {
        options.JsonSerializerOptions.PropertyNamingPolicy = new UppercaseNamingPolicy();
    });

By configuring .AddJsonOptions() inside AddControllers(), the UppercaseNamingPolicy will be applied globally, ensuring that all property names in the JSON response are uppercase.

What if I want to use PascalCase instead?

.NET also offers built-in JsonNamingPolicy for PascalCase, so you don't need to implement it yourself:

builder.Services.AddControllers()
    .AddJsonOptions(options =>
    {
        options.JsonSerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.PascalCase;
    });

In .NET 8, built-in support is available for several naming conventions, including CamelCase (camelCase), KebabCase (both lowercase kebab-case and uppercase KEBAB-CASE), and SnakeCase (both lowercase snake_case and uppercase SNAKE_CASE).

This provides a flexible and efficient way to enforce consistent naming conventions globally across your API responses without the need to manually annotate each field.