Get a list of JSON property names from a class to use in a query string

@Leigh Shepperson has the right idea; however, you can do it with less code using LINQ. I would create a helper method like this:

using System.Linq;using System.Reflection;using Newtonsoft.Json;...public static string GetFields(Type modelType){    return string.Join(",",        modelType.GetProperties()                 .Select(p => p.GetCustomAttribute<JsonPropertyAttribute>())                 .Select(jp => jp.PropertyName));}

You can use it like this:

var fields = "&fields=" + GetFields(typeof(model));

EDIT

If you're running under the 3.5 version of the .Net Framework such that you don't have the generic GetCustomAttribute<T> method available to you, you can do the same thing with the non-generic GetCustomAttributes() method instead, using it with SelectMany and Cast<T>:

    return string.Join(",",        modelType.GetProperties()                 .SelectMany(p => p.GetCustomAttributes(typeof(JsonPropertyAttribute))                                   .Cast<JsonPropertyAttribute>())                 .Select(jp => jp.PropertyName)                 .ToArray());

You can do this using reflection. This is the general idea:

using Newtonsoft.Json;using Newtonsoft.Json.Serialization;using System.Reflection;namespace ConsoleApplication8{    public class model    {        [JsonProperty(PropertyName = "id")]        public long ID { get; set; }        [JsonProperty(PropertyName = "some_string")]        public string SomeString { get; set; }    }    internal class Program    {        private static void Main(string[] args)        {            var model = new model();            var result = string.Empty;            PropertyInfo[] props = typeof(model).GetProperties();            foreach (PropertyInfo prop in props)            {                foreach (object attr in prop.GetCustomAttributes(true))                {                    result += (attr as JsonPropertyAttribute).PropertyName;                }            }        }    }}

In cases where the model is only partially annotated with [JsonProperty(PropertyName = "XXX")] attributes, or is annotated with data contract attributes, or has ignored properties, you can use Json.NET's own contract resolver to obtain the list of serialized property names. First, introduce the following extension method:

public static class JsonExtensions{    public static string [] PropertyNames(this IContractResolver resolver, Type type)    {        if (resolver == null || type == null)            throw new ArgumentNullException();        var contract = resolver.ResolveContract(type) as JsonObjectContract;        if (contract == null)            return new string[0];        return contract.Properties.Where(p => !p.Ignored).Select(p => p.PropertyName).ToArray();    }}

Then, do:

// Allocate the relevant contract resolver. // Options are CamelCasePropertyNamesContractResolver() or DefaultContractResolver().IContractResolver resolver = new DefaultContractResolver(); // Get propertiesvar propertyNames = resolver.PropertyNames(typeof(model));var fields = "&fields=" + String.Join(",", propertyNames);

For resolver use CamelCasePropertyNamesContractResolver if you are camel casing your property names (which ASP.NET Core Web API does by default); otherwise use DefaultContractResolver.

Sample fiddle.