how to exclude properties from a list of nested classes in my base class using a ContractResolver

Question

I have two classes:

public class Customer
{
    public int ID { get; set; }

    public string Name { get; set; }

    public virtual ICollection<Order> Orders { get; set; }
}

public class Order
{
    public int ID { get; set; }

    public int CustomerID { get; set; }
    public virtual Customer Customer { get; set; }

    public string Description { get; set; }
}

I am trying to serialize a Customer to JSON using Json.Net and a ContractResolver , excluding the Customer property from every Order . I just want to keep the Description . I don't want to use Annotation like [JsonIgnore] because I want to make the serialization different for different cases. So I do something like this:

public class CustomerContractResolver : DefaultContractResolver
{
    protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
    {
        JsonProperty property = base.CreateProperty(member, memberSerialization);
        if (property.DeclaringType == typeof(Order) && property.PropertyName.Equals(nameof(Order.Customer)))
        {
            property.ShouldSerialize = instance => { return false; };
        }
        return property;
    }
}

Then I get a customer from the database and try to serialize it:

JsonSerializerSettings settings = new JsonSerializerSettings { ContractResolver = new CustomerContractResolver() };
string json = Newtonsoft.Json.JsonConvert.SerializeObject(customer, Formatting.Indented, settings);

The CreateProperty method of the CustomerContractResolver is called as expected for each Order in the Orders collection, but I still get an error:

Self referencing loop detected for property 'Customer' with type 'System.Data.Entity.DynamicProxies.Customer_04661D0875E326DF9B5D4D2BA503FC19E2C0EBD49A7BE593D66F67AE77F7FDBE'. Path 'Orders[0]'.

Meaning that the ShouldSerialize delegate didn't work.

Any ideas here how to exclude the Customer property from each Order ?

Answer 1

From the exception error message it appears you are using Entity Framework and have enabled dynamic proxies :

When creating instances of POCO entity types, the Entity Framework often creates instances of a dynamically generated derived type that acts as a proxy for the entity.

Because the objects passed in are derived, the check property.DeclaringType == typeof(Order) will fail. Instead you can follow use one of the answers from unexpected GetType() result for entity entry to check whether the underlying type is Order , for instance:

public class CustomerContractResolver : DefaultContractResolver
{
    protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
    {
        JsonProperty property = base.CreateProperty(member, memberSerialization);
        if (typeof(Order).IsAssignableFrom(property.DeclaringType) && property.PropertyName.Equals(nameof(Order.Customer)))
        {
            property.ShouldSerialize = instance => { return false; };
        }
        return property;
    }
}

Checking whether the property's declaring type is assignable from Order should work whether or not dynamic proxies are being used.