How do I serialize an object into query-string format?

Building on the good ideas from other comments, I have made a generic extension method .ToQueryString(), which can be used on any object.

public static class UrlHelpers
{
    public static string ToQueryString(this object request, string separator = ",")
    {
        if (request == null)
            throw new ArgumentNullException("request");

        // Get all properties on the object
        var properties = request.GetType().GetProperties()
            .Where(x => x.CanRead)
            .Where(x => x.GetValue(request, null) != null)
            .ToDictionary(x => x.Name, x => x.GetValue(request, null));

        // Get names for all IEnumerable properties (excl. string)
        var propertyNames = properties
            .Where(x => !(x.Value is string) && x.Value is IEnumerable)
            .Select(x => x.Key)
            .ToList();

        // Concat all IEnumerable properties into a comma separated string
        foreach (var key in propertyNames)
        {
            var valueType = properties[key].GetType();
            var valueElemType = valueType.IsGenericType
                                    ? valueType.GetGenericArguments()[0]
                                    : valueType.GetElementType();
            if (valueElemType.IsPrimitive || valueElemType == typeof (string))
            {
                var enumerable = properties[key] as IEnumerable;
                properties[key] = string.Join(separator, enumerable.Cast<object>());
            }
        }

        // Concat all key/value pairs into a string separated by ampersand
        return string.Join("&", properties
            .Select(x => string.Concat(
                Uri.EscapeDataString(x.Key), "=",
                Uri.EscapeDataString(x.Value.ToString()))));
    }
}

It will also work for objects that have properties of the type Array and generic Lists if they only contain primitives or strings.

Try it out, comments are welcome: Serialize object into a query string with Reflection

I'm 99% sure there's no built-in utility method for this. It's not a very common task, since a web server doesn't typically respond with a URLEncoded key/value string.

How do you feel about mixing reflection and LINQ? This works:

var foo = new EditListItemActionModel() {
  Id = 1,
  State = 26,
  Prefix = "f",
  Index = "oo",
  ParentID = null
};

var properties = from p in foo.GetType().GetProperties()
                 where p.GetValue(foo, null) != null
                 select p.Name + "=" + HttpUtility.UrlEncode(p.GetValue(foo, null).ToString());

// queryString will be set to "Id=1&State=26&Prefix=f&Index=oo"                  
string queryString = String.Join("&", properties.ToArray());

Update:

To write a method that returns the QueryString representation of any 1-deep object, you could do this:

public string GetQueryString(object obj) {
  var properties = from p in obj.GetType().GetProperties()
                   where p.GetValue(obj, null) != null
                   select p.Name + "=" + HttpUtility.UrlEncode(p.GetValue(obj, null).ToString());

  return String.Join("&", properties.ToArray());
}

// Usage:
string queryString = GetQueryString(foo);

You could also make it an extension method without much additional work

public static class ExtensionMethods {
  public static string GetQueryString(this object obj) {
    var properties = from p in obj.GetType().GetProperties()
                     where p.GetValue(obj, null) != null
                     select p.Name + "=" + HttpUtility.UrlEncode(p.GetValue(obj, null).ToString());

    return String.Join("&", properties.ToArray());
  }
}

// Usage:
string queryString = foo.GetQueryString();