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

  • Artikkeli

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