.NET DefaultValue attribute

Question

I've heard people say a few different things about the DefaultValue attribute including:

• "It sets the value of the property before anything else uses it."
• "It doesn't work for autoproperties."
• "It's only for decoration. You must manually set actual default values."

Which (if any) is right? Does DefaultValue actually set default values? Are there cases where it doesn't work? Is it best just not to use it?

Answer 1

The place where I typically used DefaultValue is for classes which are serialized/deserialized to XML. It does not set the default value during instantiation and doesn't impact autoproperties.

From MSDN:

A DefaultValueAttribute will not cause a member to be automatically initialized with the attribute's value. You must set the initial value in your code.

MSDN - DefaultValueAttribute Class

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Edit: As Roland points out, and as others mentioned in their answers the attribute is also used by the Form Designer

Answer 2

"It sets the value of the property before anything else uses it." --> No the default value is only for the designer. a default value will not be seralized into the designer code.

"It doesn't work for autoproperties." --> No

"It's only for decoration. You must manually set actual default values." --> No. Because of the Designer Serialization. But you must set it manually.

Answer 3

Like all attributes, it's meta data, and as such "It's only for decoration. You must manually set actual default values." is closest.

MSDN goes on to say about DefaultValueAttribute:

A DefaultValueAttribute will not cause a member to be automatically initialized with the attribute's value. You must set the initial value in your code.

i.e. You still need to make the constructor match what you say is the default value so that code that trusts it still works, such as the built in XML Serialization will use them to work out whether to serialise the property or not; similarly the form designer will use the DefaultValues to work out what code needs to be automatically generated.

Answer 4

From MSDN help:

AttributeCollection^ attributes = TypeDescriptor::GetProperties( this )[ "MyProperty" ]->Attributes;

/* Prints the default value by retrieving the DefaultValueAttribute 
      * from the AttributeCollection. */
DefaultValueAttribute^ myAttribute = dynamic_cast<DefaultValueAttribute^>(attributes[ DefaultValueAttribute::typeid ]);
Console::WriteLine( "The default value is: {0}", myAttribute->Value );

I need to set the default value attribute to something non static. How can I basically set it whenever I want to?

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Solved by overriding the ShouldSerialize functions in the class holding the properties.

Example:

property System::String^ Z {
            System::String^ get(){...}
            void set(System::String^ value) {...}
        }

        bool ShouldSerializeZ() {
            return Z != <call to run time objects>
        }