Why can't a sub implement an interface and handle event at the same time?

Question

Why can't a sub implement an interface and handle event at the same time?

The following gives my a syntax error:

Sub MySub() Handles MyButton.Click Implements MyInterface.MyMethod
End Sub

I know I could handle this kind of logic with another method, but that is not the point. I just want to understand the reasoning behind this.

Answer 1

The syntax error is consistent with the language grammar, in §9.2.1 of the VB language specification¹:

SubDeclaration ::=
[ Attributes ] [ ProcedureModifier+ ] SubSignature [ HandlesOrImplements ] LineTerminator
Block
End Sub StatementTerminator

and

HandlesOrImplements ::= HandlesClause | ImplementsClause

So only one is supported on any one method. The specification doesn't (on a quick view) include a rationale for this limitation. For that you'll need to talk to someone on the VB language design team at Microsoft.

---

¹ This is included with an installation of VS under ‹VSRoot›\VB\Specifications\1033.