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.NET Forum / .NET Framework / New Users / October 2006nullptr for a property
Hi, I would like to define the default value of my property. My property is from type Bitmap^, therefore i was thinking to do : [DefaultValueAttribute(nullptr)] but i does not work. how is it possible to define this property to no pointer ? thx. ALain here is my code : #pragma region public Property : ImageSortAscendant // Allow user to select the image which will be displayed for column sorted ascendantly [System::ComponentModel::DefaultPropertyAttribute (nullptr)] property Bitmap^ ImageSortAscendant { Bitmap^ get() { return m_ImageSortA; } void set(System::Drawing::Bitmap^ value) { m_ImageSortA = value; } } #pragma endregion but when i try to compile, i have the following error : ..\RAF_ListView.h(177) : error C3115: 'System::ComponentModel::DefaultPropertyAttribute': this attribute is not allowed on 'RAF_ListView::AR_ListView::ImageSortAscendant' how could it be possible ? thanks, Al. > Hi, > [quoted text clipped - 9 lines] > > ALain > here is my code : > [quoted text clipped - 22 lines] > > how could it be possible ? It's not necessary. If you simply don't assign a value to the property (or the private field that backs it up), then it WILL be null. The DefaultProperty attribute is applied to a class and names the default property of the class. It does not provide a default value for a property, which is apparently what you're trying to do. -cd He might be trying to set the default value of the property if he is planning on designing the class. It won't default the value when the instance is created but it will allow the designer to reset the value of the property and exclude the redundant value when the instance is serialized to xml or codedom. Bryan Phillips MCSD, MCDBA, MCSE Blog: http://bphillips76.spaces.live.com > > here is my code : > > [quoted text clipped - 31 lines] > > -cd Thanks Carl, but i went to this direction according to my other issue... when i assigned a bitmap to this property, it's ok and everything works. However, once a bitmap is assigned, and i want to delete this assignment, i usually use select the property value in property editor and press delete key... it clear the property field value and it should display (none) as usual... but nothing happens... since i assign a bitmap, i can not have anymore this property field value to (none). I can only assign another bitmap, but never clear the field :-( do you have an idea ? thx. Al. >> here is my code : >> [quoted text clipped - 31 lines] > > -cd You could try the DefaultValueAttribute instead... > Thanks Carl, but i went to this direction according to my other issue... > [quoted text clipped - 49 lines] >> >> -cd My problem is not to assign a value to this property. I think it it how to clear it, if user press DELETE, BACKSPACE keys when this property has the focus in the property editor. For example if i assign a bitmap to form thanks backgroundimage property, just by pressing delete, this property is cleared. i would like to have the same for my property.. that's all Al. > You could try the > DefaultValueAttribute instead... [quoted text clipped - 51 lines] >>> >>> -cd Well why don't you try the "DefaultValueAttribute" as I told you? I think it's exactky what you need! Anyway, FYI, here is what reflector says about Control.BackgroundImage: [System::ComponentModel::DefaultValue(*static_cast<__box System::String*>(0)), System::ComponentModel::Localizable(true), System::Windows::Forms::SRDescription(S"ControlBackgroundImageDescr"), System::Windows::Forms::SRCategory(S"CatAppearance")] public: __property virtual System::Drawing::Image __gc* get_BackgroundImage() { return *static_cast<__box System::Drawing::Image*>(this->Properties->GetObject(System::Windows::Forms::Control::PropBackgroundImage)); }public: __property virtual void __gc* set_BackgroundImage(System::Drawing::Image __gc* value) { if (this->BackgroundImage != value) { this->Properties->SetObject(System::Windows::Forms::Control::PropBackgroundImage, value); this->OnBackgroundImageChanged(System::EventArgs::Empty); } } > My problem is not to assign a value to this property. I think it it how to > clear it, if user press DELETE, BACKSPACE keys when this property has the [quoted text clipped - 64 lines] >>>> >>>> -cd i tried but it does not work as expected :-( when i see the code you wrote, it seems that an event is also added. is it like that for all properties ? because i've not seen it till now. moreover, where did you find the code below ? in help ? i did not find it.. thx. Al. > Well why don't you try the "DefaultValueAttribute" as I told you? > I think it's exactky what you need! [quoted text clipped - 88 lines] >>>>> >>>>> -cd Google: Must have .NET developer tools => http://msdn.microsoft.com/msdnmag/issues/04/07/MustHaveTools/ Google: Reflector .NET => http://www.aisto.com/roeder/dotnet/ Reflector is the tool which will give you the source code from an MSIL. >i tried but it does not work as expected :-( > when i see the code you wrote, it seems that an event is also added. [quoted text clipped - 103 lines] >>>>>> >>>>>> -cd Thanks for information but after your previous post, i got it ;-) however, i would like to understand something. It seems that everything is written with CLI managed v1, while i use v2. therefore, as i do not want to use v1, i do not have /clr:OldSyntax as compiler option. I tried to migrate the code from v1 to v2 of CLI managed, but without success. I still have the same issue. I can not clear my property field value by pressing DELETE or BACKSPACE keys. Something must be wrong in my code :-( or i miss the point... Al. > Google: Must have .NET developer tools > => http://msdn.microsoft.com/msdnmag/issues/04/07/MustHaveTools/ [quoted text clipped - 111 lines] >>>>>>> >>>>>>> -cd Do not worry about the syntax! It might very well have been written in C# to start with, it's just reflector which decompile in old MC++ syntax! Glad you solved your problem. > Thanks for information but after your previous post, i got it ;-) > [quoted text clipped - 127 lines] >>>>>>>> >>>>>>>> -cd I've found where was the problem... at least :-) i should cast my default value to String as follow : System::ComponentModel::DefaultValueAttribute(static_cast <System::String^> (nullptr))] and not to an image^ ... like that it allow DELETE and BACKSPACE keys to be used. thx a lot Lloyd ! Alain > Google: Must have .NET developer tools > => http://msdn.microsoft.com/msdnmag/issues/04/07/MustHaveTools/ [quoted text clipped - 111 lines] >>>>>>> >>>>>>> -cd Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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