C# – Casting interfaces for deserialization in JSON.NET

cjsonjson.netnet

I am trying to set up a reader that will take in JSON objects from various websites (think information scraping) and translate them into C# objects. I am currently using JSON.NET for the deserialization process. The problem I am running into is that it does not know how to handle interface-level properties in a class. So something of the nature:

public IThingy Thing

Will produce the error:

Could not create an instance of type IThingy. Type is an interface or abstract class and cannot be instantiated.

It is relatively important to have it be an IThingy as opposed to a Thingy since the code I am working on is considered sensitive and unit testing is highly important. Mocking of objects for atomic test scripts is not possible with fully-fledged objects like Thingy. They must be an interface.

I've been poring over JSON.NET's documentation for a while now, and the questions I could find on this site related to this are all from over a year ago. Any help?

Also, if it matters, my app is written in .NET 4.0.

Best Answer

@SamualDavis provided a great solution in a related question, which I'll summarize here.

If you have to deserialize a JSON stream into a concrete class that has interface properties, you can include the concrete classes as parameters to a constructor for the class! The NewtonSoft deserializer is smart enough to figure out that it needs to use those concrete classes to deserialize the properties.

Here is an example:

public class Visit : IVisit
{
    /// <summary>
    /// This constructor is required for the JSON deserializer to be able
    /// to identify concrete classes to use when deserializing the interface properties.
    /// </summary>
    public Visit(MyLocation location, Guest guest)
    {
        Location = location;
        Guest = guest;
    }
    public long VisitId { get; set; }
    public ILocation Location { get;  set; }
    public DateTime VisitDate { get; set; }
    public IGuest Guest { get; set; }
}

Related Solutions

C# – Direct casting vs ‘as’ operator

string s = (string)o; // 1

Throws InvalidCastException if o is not a string. Otherwise, assigns o to s, even if o is null.

string s = o as string; // 2

Assigns null to s if o is not a string or if o is null. For this reason, you cannot use it with value types (the operator could never return null in that case). Otherwise, assigns o to s.

string s = o.ToString(); // 3

Causes a NullReferenceException if o is null. Assigns whatever o.ToString() returns to s, no matter what type o is.

Use 1 for most conversions - it's simple and straightforward. I tend to almost never use 2 since if something is not the right type, I usually expect an exception to occur. I have only seen a need for this return-null type of functionality with badly designed libraries which use error codes (e.g. return null = error, instead of using exceptions).

3 is not a cast and is just a method invocation. Use it for when you need the string representation of a non-string object.

C# Interfaces. Implicit implementation versus Explicit implementation

Implicit is when you define your interface via a member on your class. Explicit is when you define methods within your class on the interface. I know that sounds confusing but here is what I mean: IList.CopyTo would be implicitly implemented as:

public void CopyTo(Array array, int index)
{
    throw new NotImplementedException();
}

and explicitly as:

void ICollection.CopyTo(Array array, int index)
{
    throw new NotImplementedException();
}

The difference is that implicit implementation allows you to access the interface through the class you created by casting the interface as that class and as the interface itself. Explicit implementation allows you to access the interface only by casting it as the interface itself.

MyClass myClass = new MyClass(); // Declared as concrete class
myclass.CopyTo //invalid with explicit
((IList)myClass).CopyTo //valid with explicit.

I use explicit primarily to keep the implementation clean, or when I need two implementations. Regardless, I rarely use it.

I am sure there are more reasons to use/not use explicit that others will post.

See the next post in this thread for excellent reasoning behind each.

Best Answer

Related Solutions

C# – Direct casting vs ‘as’ operator

C# Interfaces. Implicit implementation versus Explicit implementation

Related Topic