"is" and "as"

[TonyValenti]

I have always understood the following code snippets to be equivalent:

//Method 1
object o = ...;
var x = o as Interface;
if(x != null){
  var y = x;
  ...
}

and:

//Method 2
object o = ...;
if(o is Interface y){ 
  ...
}

However, I recently ran into a situation involving COM interfaces where Method1 would yield true but Method2 would yield false.

How are is and as related, specifically as it relates to COM interfaces?

[TonyValenti]

Err. Minor error in code snipped. Fixed online.

[CyrusNajmabadi]

However, I recently ran into a situation involving COM interfaces where Method1 would yield true but Method2 would yield false.

It might be interesting to see the different IL you're seeing in this case.

[HaloFour]

Unless the C# compiler is emitting different IL specifically for COM interfaces they should be roughly equivalent. A quick&dirty example on SharpLab yielded very similar IL:

Method 1:

        IL_0000: ldarg.1
        IL_0001: isinst Foo
        IL_0006: stloc.0
        IL_0007: ldloc.0
        IL_0008: brfalse.s IL_0011

        IL_000a: ldloc.0

Method 2:

        IL_0000: ldarg.1
        IL_0001: isinst Foo
        IL_0006: dup
        IL_0007: stloc.0
        IL_0008: brfalse.s IL_0011

        IL_000a: ldloc.0

[RikkiGibson]

This looks like a compiler bug, it would be greatly appreciated if you could provide a reproducer and post it as an issue in https://github.com/dotnet/roslyn

[yaakov-h]

FWIW I can't reproduce this using CERTENROLLLib.IAlternativeName as the interface. I'm seeing identical IL in both scenarios.

[TonyValenti]

I'll try to put together an example later today.

On Sat, Sep 21, 2019 at 5:53 AM Yaakov ***@***.***> wrote: FWIW I can't reproduce this using CERTENROLLLib.IAlternativeName as the interface. I'm seeing identical IL in both scenarios. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <https://github.com/dotnet/csharplang/issues/2815?email_source=notifications&email_token=ADH2UVQCWY4KRDIOPVMZOO3QKX4MLA5CNFSM4IY2YNF2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOD7IPNSQ#issuecomment-533788362>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ADH2UVX22FXGFNS4WQIP5LLQKX4MLANCNFSM4IY2YNFQ> .

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