'Matching return types when returning an implementation of multiple interfaces

How can I match return types when returning an implementation of multiple interfaces if each of those interfaces is required by counterpart interface implementations in a single function?

That's my best shot at trying to put this into words, onto the code:

interface IA
{
    IAA XYZ();
}

interface IAA
{

}

interface IB
{
    IBB XYZ();
}

interface IBB
{

}

class IAABB : IAA, IBB
{

}

class T : IA, IB
{
    public IAABB XYZ() { return null; }
}

The code above results in "T.XYZ() cannot implement IA.XYZ() because it does not have the matching return type of IAA". Same for IB.XYZ(), of course.

c++sharpinheritanceinterfacecovariance


Solution 1:[1]

You can do this by implementing interfaces explicitly:

class T :IA,IB{
    IAA IA.XYZ() { return XYZ(); }
    IBB IB.XYZ() { return XYZ(); }
    public IAABB XYZ() { return null; }

this will work only if IAA or IBB is an interface and not classes because you cannot inherit class IAABB from more than one base class.

Sources

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Source: Stack Overflow

Solution Source
Solution 1 Rafal

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