Deconstruct Extension Method ignored when type already has Deconstruction method(s).

[obiwanjacobi]

I always thought that picking the Deconstruction method was done by using it's parameter types. However I recently ran into a problem that seems to indicate otherwise.

Concreet: I wanted an extra Deconstruct method on DateTime. My Deconstruct method has unique parameter types, so there can be no mistakes/ambiguity. However, as I set out to write my unit test, the compiler insisted on using the native DataTime.Deconstruct (with int params) and not my version.

Years, Months and Days are my custom types, all declared as readonly record struct:

public static void Deconstruct(this DateTime dateTime, out Years years, out Months months, out Days days)
{ /* assign output param values here */}

var dt = new DateTime(....);
(Years years, Months months, Days days)  = dt;  // <= error - not using my Deconstruct version

I even tried to use the OverloadResolutionPriorityAttribute but that didn't fix it.

Q:

• How does the resolution work? Is it a simple matter that extensions are ignore when a native Deconstruct method is detected?
• Why was it designed that way / what are the problems with looking at parameter types?

[iam3yal]

When using extension methods the extension method needs to be marked with static.

[obiwanjacobi]

It was - I just 'copied' it wrong... Thanks.

[HaloFour]

See: #1092