attribute-parameter-types-cpp-component-extensions.md

description	Learn more about: Attribute Parameter Types (C++/CLI and C++/CX)
title	Attribute Parameter Types (C++/CLI and C++/CX)
ms.date	10/12/2018
ms.topic	reference
helpviewer_keywords	custom attributes, parameter types
ms.assetid	d9f127a3-7f08-456f-acc6-256805632712

Attribute Parameter Types (C++/CLI and C++/CX)

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: Attribute parameter types

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: Unnamed arguments precede named arguments

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: One-dimensional array attribute parameter

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