How to access class methods after institiating object as interface?

Question

What I would like to do is to have an interface like IResult result = new SomeResult(), and then depending on some if's, access some specific fields after casting.

if (a == b) 
{
   result = (SomeOtherResult) result;
   result.fieldFromSomeOtherResult = 42;
}

Obviously now I can't do that, because result's interface doesn't have this field, neither does SomeResult class, only SomeOtherResult class. How should I solve this?

How does IResult, SomeResult and SomeOtherResult related to each other? — Pavel Anikhouski
– Pavel Anikhouski, Commented May 5, 2020 at 10:50
Being more specific (why IResult, what field, what are you trying, etc.) will help to get specific to your case answers. There could be an existing pattern already. Maybe use generics to pass SomeOtherResult type or perhaps it shouldn't be IResult, but a more suitable interface (implemented by SomeOtherResult). — Sinatr
– Sinatr, Commented May 5, 2020 at 11:17
I think I approached it from a bit incorrect point of view. @Dai's answer helped me realise something, so I've checkmarked it. — DisplayMyName
– DisplayMyName, Commented May 5, 2020 at 11:19

Dai · Accepted Answer · 2020-05-05 10:51:21Z

4

If you're using C# 7.0 or later, use the is operator:

IResult result = new SomeResult();

if( a == b )
{
    if( result is SomeResult someResult )
    {
        someResult.fieldFromSomeOtherResult = 42;
    }
    else if( result is SomeOtherResult someOtherResult )
    {
        someOtherResult.x = 123;
    }
}

That said, if you're initializing IResult result using an explicit new SomeResult() then you should declare result with the SomeResult type.

answered May 5, 2020 at 10:51

Dai

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2 Comments

Pavel Anikhouski Over a year ago

To be precise, is operator for type checking was exist before C# 7, C# 7 introduced a pattern matching with is operator

Dai Over a year ago

@PavelAnikhouski I don't like how the C# <del>marketing material</del> documentation refers to it as "pattern matching" because it makes me think of Haskell matching or otherwise implies (to me, at least) that it's an O(1) operation regardless of the size of the match's candidate expression list - when in reality it's an O(n) ordered sequence of type-checks - and that's all it is. I don't think it's possible for a language to have true pattern-matching without structural typing and value-typing (i.e. each distinct value, or set of values constitutes its own separate type) which C# lacks.

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