TodoCollectionConverter.cs

public class TodoCollectionConverter : IStateConverter
{
    public Task<object> ConvertAsync(StateConverterContext context)
    {
        var todos = (context.Object as IEnumerable<TodoModel>);
        var path = context.HttpContext.Request.GetDisplayUrl();

        var properties = new
        {
            count = todos.Count()
        };

        var document = new Document
        {
            Class = new Class {"todo", "collection"},
            Properties = properties,
            Href = path,
        };

        var entities = todos.Select(a => new Entity
        {
            Class = new Class {"todo"},
            Properties = a,
            Href = $"{path}/{a.TodoId}",
        });

        document.Entities.Add(entities);

        return Task.FromResult<object>(document);
    }
}