C# Covariant Returns for Auto-Implemented Properties

Question

As you know, C# 9.0 (.Net 5) now allows Covariant Returns. I need help applying this to a set of classes having Auto-Implemented properties.

I have two abstract classes that represent financial bank accounts and transactions. I made them abstract since I will pull data from various data sources and while the main properties will be common across all sources, each source may have additional fields I want to keep. A 1 to Many relationship exists between both classes (1 account has many transactions AND 1 transaction belongs to only 1 account).

public abstract class BankAccount
{
    public string Name { get; set; }
    public IList<Transaction> Transactions { get;  set; } = new List<Transaction>();
    ...
}

public abstract class Transaction
{
    public string Name { get; set; }

    public virtual BankAccount BankAccount { get; set; } // This doesn't work unless I remove set;
    ...
}

And here is an example of the concrete implementations

public class PlaidBankAccount : BankAccount
{
    public string PlaidId { get; set; }
    ...

}
public class PlaidTransaction : Transaction
{
    public string PlaidId { get; set; }
    public override PlaidBankAccount BankAccount { get; set; } // This doesn't work unless I remove set;
    ...
}

What I want to do is to override the base class getters and setters so that they use derived classes. For example:

If I create an instance of the concrete transaction and call the BankAccount getter, I want to get an instance of the derived PlaidBankAccount not the base BankAccount.

What I've found is that when I only define virtual getter in the base class and override it in the derived class, it works. But just as I add both properties {get;set;}, I get the same error as in previous C# versions:

error CS1715: 'PlaidTransaction.BankAccount': type must be 'BankAccount' to match overridden member 'Transaction.BankAccount'

How could I fix this?

Answer 1

In C# 9 properties are only able to have co-variant returns when they are readonly, so unfortunately, no set; is possible.

An overriding property declaration must specify exactly the same access modifier, type, and name as the inherited property. Beginning with C# 9.0, read-only overriding properties support covariant return types. The overridden property must be virtual, abstract, or override.

From the Microsoft Docs - Override keyword