Anonymous Interface Objects

[x] Proposed
[ ] Prototype: Not Started
[ ] Implementation: Not Started
[ ] Specification: Not Started

Summary

Give C# developers the ability to instantiate anonymous objects that implement a specific interface without first implementing a concrete type.

Motivation

In scenarios where developers need to stub a type for unit tests, it becomes increasingly tedious to create classes that are essentially throw away. They add to the complexity of the project and ultimately increase the noise/signal ratio.

Detailed design

public interface IClient
{
    string Name { get; }
}

// single interface
var client = new IClient {
    Name = "Khalid"
};

We can also implement multiple interfaces on an anonymous type.

public interface IClient
{
    string Name { get; }
}

public interface ICustomer
{
    string AccountNumber { get; }
}

// multiple interface
var client = new IClient, ICustomer {
    Name = "Khalid",
    AccountNumber = "8675309"
};

Method implementations can be implemented using lambda methods.

public interface ICashier
{
    decimal CheckOut(Cart cart);
}

var cashier = new ICashier {
   CheckOut = (c) => cart.Total() * 0.2d;
}

Drawbacks

There are ways to address this issue already in the C# language that requires reflection.

Alternatives

Create stub types for every new scenario you need an interface.

Use Kotlin. Here is a working example where the khalid instance is an anonymous object that happens to implement the IGreeting interface.

    interface IGreeting {
        val name: String
    }

    class Default(override val name: String) : IGreeting

    fun main() {

        val csharp = Default("C# Developer")
        // an anonymous object
        // that implements IGreeting
        val khalid = object: IGreeting {
            override val name: String
                get() = "Khalid (You're Awesome)"
        }

        var folks = listOf(csharp, khalid)
        folks.forEach { println("Hello ${it.name}!") }
    }

Unresolved questions

What is the behavior when interfaces have conflicting properties/methods?
Can you define explicit interface implementations?

dotnet / csharplang