DeliminatorSeparatedPropertyNamesContractResolver.cs

public class DeliminatorSeparatedPropertyNamesContractResolver : DefaultContractResolver
{
    private readonly string _separator;

    protected DeliminatorSeparatedPropertyNamesContractResolver(char separator) : base(true)
    {
        _separator = separator.ToString();
    }

    protected override string ResolvePropertyName(string propertyName)
    {
        var parts = new List<string>();
        var currentWord = new StringBuilder();

        foreach (var c in propertyName)
        {
            if (char.IsUpper(c) && currentWord.Length > 0)
            {
                parts.Add(currentWord.ToString());
                currentWord.Clear();
            }
            currentWord.Append(char.ToLower(c));
        }

        if (currentWord.Length > 0)
        {
            parts.Add(currentWord.ToString());
        }

        return string.Join(_separator, parts.ToArray());
    }
}

SnakeCasePropertyNamesContractResolver.cs

public class SnakeCasePropertyNamesContractResolver : DeliminatorSeparatedPropertyNamesContractResolver
{
    public SnakeCasePropertyNamesContractResolver() : base('_') { }
}