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How do I implement Post([FromBody] IMessage msg) in my .Net Core 2.1 web api Controller?

I get the following exception when I send a simple JSON structure implementing the interface IMessage:

Could not create an instance of type TestService2.IMessage. Type is an interface or abstract class and cannot be instantiated. Path 'Id', line 2, position 7.

1 Answer
1

I absolutely agree that using classes is the way to go!

If, however, for some obscure/Stupid reason, you have to use interfaces like me, there is a simple way of mapping it to a class with JSON.

In the Startup Method, define the Contracts for the interface:

services.AddMvc()
.AddJsonOptions(o => { o.SerializerSettings.ContractResolver.ResolveContract(typeof(IMessage)).Converter = new MyJsonConverter<IMessage, Message>();})


Where IMessage is said interface, and MyJsonConverter is derived from JsonConverter, for example:

public class MyJsonConverter<Tin, Tout> : JsonConverter
{
public override bool CanConvert(Type objectType)
{
//Your logic here
return (objectType == typeof(Tin) );
}

public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
//deserializing from the implementation Class
return serializer.Deserialize<Tout>(reader);
}

public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
//use standard serialization
serializer.Serialize(writer, value);
}
}


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