为什么我的ICollection项 <Interface> WebAPI调用是否为null？

Question

I am making a WebAPI call to get an object back similar to this one:

public class DocumentCategoryModel : IDocumentCategoryTree
    {
        public DocumentCategoryModel()
        {
            SubCategories = new List<IDocumentCategoryTree>();
        }

        public int ID { get; set; }
        public ICollection<IDocumentCategoryTree> SubCategories { get; set; }
    }

The ID and other properties are populated correctly. The SubCategories prop has a collection of the correct size, but every entry is null.

Any idea why? I can't make it a collection of a concrete type (out of my control), but the WebAPI is able to figure out which class it should be...

Answer 1

This answer assumes you're using JSON.net which is the default with ASP.NET WebAPI. Since JSON.net doesn't understand how to create a concrete instance of IDocumentCategoryTree it cannot deserialize that automatically for you. So you will need to do this youself, the way I would do this is using a JsonConverter for your class:

public class DocumentCategoryModel : IDocumentCategoryTree
{
    public DocumentCategoryModel()
    {
        SubCategories = new List<IDocumentCategoryTree>();
    }

    public int ID { get; set; }

    [JsonConverter(typeof(DocumentCategoryTreeConverter)]
    public ICollection<IDocumentCategoryTree> SubCategories { get; set; }
}

public class DocumentCategoryTreeConverter : JsonConverter
{
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        // Todo if desired
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        return serializer.Deserialize<TestTree>(reader);
    }

    public override bool CanConvert(Type objectType)
    {
        // Todo
    }
}

This is a very simplistic and incomplete example to give you an idea how you can do this and get you started. You can read more about JsonConverter s in some other SO posts starting here: How to implement custom JsonConverter in JSON.NET to deserialize a List of base class objects? .

Answer 2

XML and JSON serialization/deserializtion should be happening with concrete classes. If it doesn't have a concrete it won't know how to properly serialize it. I suggest creating a concrete for IDocumentCategoryTree , something like DocumentCatTreeApiModel . That will allow for the serialization process to work properly.