C++/CLI: Attributes

Predefined .NET Attributes

Many attributes are provided with the .NET implementation; however, not all of them are included in the CLI Standard. They are all derived directly from System::Attribute, which, in turn, is derived directly from System::Object. Some examples are:

System::CLSCompliantAttribute
System::Diagnostics::ConditionalAttribute
System::FlagsAttribute

System::NonSerializedAttribute
System::ObsoleteAttribute
System::ParamArrayAttribute


System::Runtime::InteropServices::DllImportAttribute
System::Runtime::InteropServices::FieldOffsetAttribute
System::Runtime::InteropServices::InAttribute

System::Runtime::InteropServices::OutAttribute
System::SerializableAttribute
System::ThreadStaticAttribute

Enum Value Formatting

In a previous installment, the following reader exercise was proposed: "Many enum types are defined simply to have a set of named constants with distinct values. Others are defined with enumerators whose initial values are explicitly initialized with distinct values that are a power of two (as in 1, 2, 4, 8, etc.). Define a CLI enum type having the latter and create an instance whose value is the bitwise-or of multiple enumerators. Display the value of that instance. Then apply the attribute [Flags] to that CLI enum type and run the program again. What is the difference? Read the documentation on the type System::FlagsAttribute." The following example is a solution to a slight variation of that problem:

using namespace System;

public enum class Color {Red = 1, Blue = 2, Yellow = 4};
[Flags] public enum class Position {Left = 1, Right = 2, Top = 4, Bottom = 8};

int main()
{
/*1*/   Console::WriteLine(Color::Red);
/*2*/   Console::WriteLine(Color::Red | Color::Blue);
/*3*/   Console::WriteLine(Color::Red | Color::Blue | Color::Yellow);

/*4*/   Console::WriteLine(Position::Left);
/*5*/   Console::WriteLine(Position::Left | Position::Right);
/*6*/   Console::WriteLine(Position::Left | Position::Right | Position::Top);
}

The output produced, is as follows:

Red
3
7

Left
Left, Right
Left, Right, Top

The CLI classifies enum constants into two categories: enumeration constants and bit-fields (which are not to be confused with C++'s own bit-fields). Quoting the CLI: "Bit-fields are generally used for lists of elements that might occur in combination; whereas enumeration constants are generally used for lists of mutually exclusive elements. Therefore, bit-fields are designed to be combined with the bitwise OR operator to generate unnamed values, whereas enumerated constants are not." Using that classification, Position's members are bit-fields while Color's are not, and the presence of the [Flags] attribute on Position's definition indicates that for certain operations, such as formatted output, combinations of enum values should be handled differently.