How to serialize as Json an object structure with circular references?

Question

I have an object structure like this:

public class Proposal {
    public List<ProposalLine> Lines { get; set; }
    public string Title { get; set; }
}

public class ProposalLine {
    public Proposal Proposal { get; set; }  // <- Reference to parent object
}

I try to serialize Proposal as Json, it tells me that there is a circular reference, which is correct.
Unfortunately, I can't touch the objects, since they are in a referenced DLL from another project - otherwise I'd change them.

Is there a way to serialize as Json and ignore the circular properties?

Solution

Use the Newtonsoft.Json (which is the default .net json serializer) and set

JsonSerializerSettings settings = new JsonSerializerSettings
{
    PreserveReferencesHandling = PreserveReferencesHandling.Objects
};
var serializer = JsonSerializer.Create(settings);

You can also globally define this variable if you are developing MVC applications...