Cannot override property with private set

[Rabadash8820]

Version Used:
Compiler version: '4.0.1-1.21568.1 (6ab66011)'.
Language version: latest (10.0).
Visual Studio version: 17.0.4

Steps to Reproduce:

1. Declare a base class with an auto-implemented property:

public class Base {
    public virtual int Member { get; private set; }
}

2. Attempt to override the property in a derived class:

public class Derived : Base {
    public override int Member { get; }    // CS8080
}

3. Same behavior and error occur whether or not Base is abstract. Declaring Derived.Member with { get; private set; } instead results in "CS0115 'Derived.Member.set': no suitable method found to override". Also, if Base.Member is declared abstract then that line fails to compile with "CS0442 'Base.Member.set': abstract properties cannot have private accessors". These latter two errors make sense but the CS8080 one seems wrong.

I am reporting this as a bug, rather than a language feature request, b/c the resulting CS8080 error seems to be almost completely undocumented. The VS link goes to the "Sorry, we don't have specifics on this C# error" page of the MS docs, a quick google turned up basically zero meaningful results, and the only mention of it in this entire repo is this unit test and this open issue from four years ago: dotnet/roslyn#23326. The code I provided in the repro steps seems like it should be valid C# to me, but if its not, then at least the documentation should be fixed.

Expected Behavior:

I am able to override auto-implemented properties declared with private set using an auto-implemented property without set.

Actual Behavior:

Overring an auto-implemented property declared with private set using an auto-implemented property without set results in undocumented CS8080 compiler error.

[jaredpar]

Transferring to docs because this behavior is intentional here. Agree there is limited docs on it. In short though if you override an auto-implemented property then you need to provide implementations of all accessors

[smargoli2]

Hi,

How should I resolve this issue? I cannot implement the private set method and I cannot leave it out. What can I do?

[adegeo]

You have to declare the property explicitly, not through an auto implemented.

class Base1 {
    public virtual int Member { get; private set; }
}

class Derived : Base1 {
    public override int Member
    {
        get => base.Member + 1;
    }
}

The csharp/language-reference/compiler-messages/property-declaration-errors.md#property-overrides section should have a tip in the bullet point for CS8080 that specifies that if the base has a private accessor, the overriding member can't be an auto-implemented property and must declare an implimentation.