Azure Functions (Durable) - Type is an interface or abstract class and cannot be instantiated

问题

Problem I have a C# Durable Azure Functions app (v2) containing an orchestration and activity function.

The orchestration calls the activity as follows:

propBag.CE = await context.CallActivityAsync<CEData>(nameof(GetCEByExternalIdActivity), propBag);

The call works ok and the Activity function executes, builds up the content for propBag.CE and returns.

The CE object contains a few other objects, including:

        public List<IParty> Individuals { get; set; }
        public List<IParty> Organisations { get; set; }

These properties are assigned to as follows:

        consPort.Individuals = await GetParties(clientRel.RelatedIndividuals, CEEntityType.Individual);
        consPort.Organisations = await GetParties(clientRel.RelatedOrganisations, CEEntityType.Organisation);

The fact that it contains an Interface seems to give the Durable Functions runtime a problem. When it attempts to deserialize the value returned from the activity function, I get the following error:

Could not create an instance of type Interfaces.Avaloq.Application.Models.CE.IParty. Type is an interface or abstract class and cannot be instantiated

Does anyone know of a good fix for this? Maybe there's a way to configure how the function will attempt to deserialize the json?

Workaround I worked-around the problem by changing the class to contain lists of the concrete types of IParty as follows:

        public List<Individual> Individuals { get; set; }
        public List<Organisation> Organisations { get; set; }

I then had to ensure that I cast to the concrete type from the IParty before assigning to those properties, as follows:

var myList = (await GetParties(clientRel.RelatedIndividuals, CEEntityType.Individual));
        foreach (var party in myList)
        {
            consPort.Individuals.Add((Individual)party);
        }
        myList = (await GetParties(clientRel.RelatedOrganisations, CEEntityType.Organisation));
        foreach (var party in myList)
        {
            consPort.Organisations.Add((Organisation)party);
        }

Not pretty but gets me around the problem.

回答1:

It looks like you're running into this issue on GitHub. In addition to your workaround, you can customize the serializer settings used by Durable Functions to include type information. There is another StackOverflow post which describes this and has an example.

来源：https://stackoverflow.com/questions/62080090/azure-functions-durable-type-is-an-interface-or-abstract-class-and-cannot-be