You might run into this one from time to time. I know it has bitten a LOT of people and it’s not intuitive at all.<\/p>\n\n\n\n
In a new desktop app add a new Class. The default name of Class1<\/em> will work fine. And create a new subclass of this. Again the default of CustomClass1<\/em> will work fine.<\/p>\n\n\n\n Then in the default window add the open event handler. In there put these few lines of code.<\/p>\n\n\n\n Then add two methods to Window1<\/p>\n\n\n\n The first should be defined as<\/p>\n\n\n\n And then we need two versions of Foo<\/p>\n\n\n\n Thats it. If you run this you might think that because methodThatCreatesSubclassInstance<\/em> returns a CustomClass1<\/em>, which you can see in the debugger at runtime, that the sub that takes a CustomClass1<\/em> would be the one that is called.<\/p>\n\n\n\n And you might try various tricks to make that happen like changing the return type of methodThatCreatesSubclassInstance<\/em> to be CustomClass1<\/em>.<\/p>\n\n\n\n Short of explicitly casting, which you really might want to avoid, you’ll find that the version of Foo<\/em> that takes a Class1<\/em> parameter is the one that gets called. <\/p>\n\n\n\n And this is something I’ve long considered a bug. But various compiler developers have disagreed \ud83d\ude41<\/p>\n\n\n\n The important thing here is not the runtime type that the variable c<\/em> holds. At runtime you can inspect it and see that it has exactly what we expected – an instance of CustomClass1<\/em>.<\/p>\n\n\n\n What IS important is the declared type <\/strong>of c – a Class1<\/em>. And the compiler decides, at compile time, that the version of Foo<\/em> that should be used is the one that has a Class1<\/em> parameter.<\/p>\n\n\n\n Careful out there.<\/p>\n","protected":false},"excerpt":{"rendered":" You might run into this one from time to time. I know it has bitten a LOT of people and it’s not intuitive at all. In a new desktop app add a new Class. The default name of Class1 will work fine. And create a new subclass of this. Again the default of CustomClass1 will … <\/p>\nDim c As Class1\nc = methodThatCreatesSubclassInstance\nfoo(c)<\/code><\/pre>\n\n\n\nPrivate Function methodThatCreatesSubclassInstance() as Class1\n Return New CustomClass1\nEnd Function\n<\/code><\/pre>\n\n\n\nPrivate Sub foo(c as Class1)\n Break\nEnd Sub\n\nPrivate Sub foo(c as CustomClass1)\n Break \nEnd Sub\n<\/code><\/pre>\n\n\n\n