Invoking default interface method implementation from a selected interface

[damirarh]

Let's say I have the following rather contrived scenario:

interface I0
{
    string Method()
    {
        return "I0";
    }
}

interface I1 : I0
{
    string I0.Method()
    {
        return "I1";
    }
}

interface I2 : I0
{
    string I0.Method()
    {
        return "I2";
    }
}

Now I want a class to implement both I1 and I2:

class C12 : I1, I2
{
}

As expected, I get the following compiler error:

> Interface member 'I0.Method()' does not have a most specific implementation. Neither 'I1.I0.Method()', nor 'I2.I0.Method()' are most specific.

I can resolve this if I provide my own implementation. Inside it, how could I invoke the implementation from one of the interfaces, e.g. I1? I'm pretty sure the following syntax used to work in a previous preview:

class C12 : I1, I2
{
    string I0.Method()
    {
        return base(I1).Method();
    }
}

However, now the compiler complains:

> Use of keyword 'base' is not valid in this context

What is the correct syntax to invoke the default method implementation in interface I1? I can't find it documented anywhere.

---

Document Details

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• ID: c790425e-da4e-9a75-6a11-f72109ce8d6a
• Version Independent ID: 09298e41-8f5a-c3c3-a849-8de67f8efba1
• Content: Safely update interfaces using default interface members in C#
• Content Source: docs/csharp/tutorials/default-interface-members-versions.md
• Product: dotnet-csharp
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

[BillWagner]

Thanks for writing this @damirarh

I'll make sure to get it updated this month.

I did a quick search, and I can't find the LDM notes where this was updated. @agocke @jcouv Can you point me to the correct updates?

[jcouv]

Yes, the error for ambiguous implementation is expected (notes).

For the base(I1) question, the functionality was removed from C# 8. Here's the PR. This was decided in the 04-29 meeting.
We have a follow-up championed issue (currently considered for C# 9).

[BillWagner]

Thanks @jcouv

I'll review the doc carefully and make sure it reflects the current planning.

[BillWagner]

I did review the tagged article, and it's correct with respect to the C# 8.0 implementation.

Continued discussion should be related to the dotnet/csharplang#2337

Closing this as by-design.