Attribute Parameter Types (C++ Component Extensions)

 

The latest version of this topic can be found at Attribute Parameter Types (C++ Component Extensions).

Values passed to attributes must be known to the compiler at compile time. Attribute parameters can be of the following types:

• bool

• char, unsigned char

• short, unsigned short

• int, unsigned int

• long, unsigned long

• __int64, unsigned __int64

• float, double

• wchar_t

• char* or wchar_t* or System::String*

• System::Type ^

• System::Object ^

• enum

Example

Code

// attribute_parameter_types.cpp  
// compile with: /clr /c  
using namespace System;  
ref struct AStruct {};  
  
[AttributeUsage(AttributeTargets::ReturnValue)]  
ref struct Attr : public Attribute {  
   Attr(AStruct ^ i){}  
   Attr(bool i){}  
   Attr(){}  
};  
  
ref struct MyStruct {  
   static AStruct ^ x = gcnew AStruct;  
   [returnvalue:Attr(x)] int Test() { return 0; }   // C3104  
   [returnvalue:Attr] int Test2() { return 0; }   // OK  
   [returnvalue:Attr(true)] int Test3() { return 0; }   // OK  
};  

Example

Description

When specifying attributes, all unnamed (positional) arguments must precede any named arguments.

Code

// extending_metadata_c.cpp  
// compile with: /clr /c  
using namespace System;  
[AttributeUsage(AttributeTargets::Class)]  
ref class MyAttr : public Attribute {  
public:  
   MyAttr() {}  
   MyAttr(int i) {}  
   property int Priority;  
   property int Version;  
};  
  
[MyAttr]   
ref class ClassA {};   // No arguments  
  
[MyAttr(Priority = 1)]   
ref class ClassB {};   // Named argument  
  
[MyAttr(123)]   
ref class ClassC {};   // Positional argument  
  
[MyAttr(123, Version = 1)]   
ref class ClassD {};   // Positional and named  

Example

Description

Attribute parameters can be one-dimensional arrays of the previous types.

Code

// extending_metadata_d.cpp  
// compile with: /clr /c  
using namespace System;  
  
[AttributeUsage(AttributeTargets::Class)]  
public ref struct ABC : public Attribute {  
   ABC(array<int>^){}  
   array<double> ^ param;  
};  
  
[ABC( gcnew array<int> {1,2,3}, param = gcnew array<double>{2.71, 3.14})]   
ref struct AStruct{};  

See Also

User-Defined Attributes