Adding snake_case support to System.Text.Json

•

Background

System.Text.Json only supports CamelCase naming policy out of the box. The good news is that support is coming but in the meantime you will need to roll your own policy. Luckily it’s really easy to do.

Solution

Newtonsoft.Json has support for snake case so we’ll borrow that for now. It has a little String utility here which can be used to convert a property name to snake_case.

Just wrap that bit of code up into a utility class.

 /// <summary>
    /// ToSnakeCase algorthim from <see href="https://github.com/JamesNK/Newtonsoft.Json/blob/7b8c3b0ed0380cf76d66894e81bf4d4d5b0bd796/Src/Newtonsoft.Json/Utilities/StringUtils.cs#L200-L276">Newtonsoft.Json</see>
    /// </summary>
    public static class StringUtils
    {
        internal enum SnakeCaseState
        {
            Start,
            Lower,
            Upper,
            NewWord
        }

        /// <summary>
        /// Convert a string to snake case.
        /// </summary>
        /// <param name="s"></param>
        /// <returns></returns>
        public static string ToSnakeCase(string s)
        {
            if (string.IsNullOrEmpty(s))
            {
                return s;
            }

            //...Code omitted for brevity...

            return sb.ToString();
        }
    }

The reference it in a naming policy.

public class SnakeCaseNamingPolicy : JsonNamingPolicy
{
    public override string ConvertName(string name)
    {
        return StringUtils.ToSnakeCase(name);
    }
}

That’s it! Snake case support is now added.

Usage

To serialise your objects in snake_case, just set the naming policy in the options.

JsonSerializer.Serialize(objectToSerialise, new JsonSerializerOptions
            {
                PropertyNamingPolicy = new SnakeCaseNamingPolicy(),
                AllowTrailingCommas = false
            });
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