JsonIgnore attributes not working in ASP.NET?

You likely have some other property that links back to its parent. Use the ReferenceLoopHandling.Ignore setting to prevent self-referencing loops.

using Newtonsoft.Json;

JsonSerializerSettings jsSettings = new JsonSerializerSettings();
jsSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore;

string json = JsonConvert.SerializeObject(foobars, Formatting.None, jsSettings);

If anyone needs an ASP.Net Core implementation of ignore child references, here it is.

public void ConfigureServices(IServiceCollection services)
{
...

    services.AddMvc()


         .AddJsonOptions(
            options => options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore
        );

    ...
}

src: https://docs.microsoft.com/en-us/ef/core/querying/related-data

I had incorrectly resolved the JsonIgnore reference.

Note that this attribute exists in more than one namespace:

• System.Text.Json.Serialization
• Newtonsoft.Json

I had resolved this in VS to System.Text.Json.Serialization.JsonIgnore - however I was using the Newtonsoft library for my actual Serialise/Deserialise - and hence the attribute was ignored. Changing the reference to Newtonsoft.Json.JsonIgnore resolved.