Mega Code Archive
An Invalid Hard Cast is NIL in Delphi for .NET. Find out why TButton(Edit1) is NIL in Delphi for .NET
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Title: An Invalid Hard-Cast is NIL in Delphi for .NET. Find out why TButton(Edit1) is NIL in Delphi for .NET
In Delphi you can (type) cast an object to a different class using either a hard-cast: TObject(Sender).Name or an as-cast (soft-cast): (Sender as Object).Name.
TObject(Sender) vs. (Sender as TObject) explains differences between an as-cast and a hard-cast from the WIN32 perspective.
Both of the supported type-casts will "convert" an object to a different type object.
The hard-cast forces an object to act as a diferent object. If the hard-cast is invalid, meaning that an object you are casting cannot be casted to the class you want it to "mimic", access violation might occur at run-time. However hard-casts are fater and when paired to the IS check a hard cast can be safe.
On the other hand, an as-cast checks if an object can be casted to a different type. If not, an "Invalid Type Cast" will be raised by Delphi.
An Invalid Hard-Cast is NIL in Delphi for .NET
Since .NET is more strict when it comes to casting an object to a different type, Delphi for .NET does not allow "unsafe" hard-casts.
In a scenario where TDeveloper extends TPerson, and TLanguage is not related to any of the two classes:
type
TLanguage = class(TObject)
TPerson = class(TObject)
TDeveloper = class(TPerson)
...
var
developer : TDeveloper;
language : TLanguage;
begin
//an ok cast
TPerson(developer).PersonMethod;
//invalid cast
language = TLanguage(developer) ;
//object reference not set to an instance of an object
language.LanguageMethod;
end
In the above code, "language" is NIL after the invalid cast - and the result is the .Net version of the access violation exception: object reference not set to an instance of an object.
Note: as-casts and is-tests work identically in Win32 and .NET.
C# Code Readers Warning
If you have some C# code you need to convert to Delphi, be very careful.
Delphi for .NET's invalid hard-cast TObject(Sender) returns NIL. C# version returns NIL when an as-cast is used.
Therefore, C#'s (Sender as TObject) should be translated to Delphi's TObject(Sender).
A confession: I have a "bug" in the title of this article! If Edit1 is a TEdit, Delphi for .NET compiler will not let Button(Edit1) to be compiled. E2089 Invalid Typecast will be raised at compile time.
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