Write custom attributes
To design custom attributes, you don't need to learn many new concepts. If you're familiar with object-oriented programming and know how to design classes, you already have most of the knowledge needed. Custom attributes are traditional classes that derive directly or indirectly from the System.Attribute class. Just like traditional classes, custom attributes contain methods that store and retrieve data.
The primary steps to properly design custom attribute classes are as follows:
This section describes each of these steps and concludes with a custom attribute example.
A custom attribute declaration begins with the System.AttributeUsageAttribute attribute, which defines some of the key characteristics of your attribute class. For example, you can specify whether your attribute can be inherited by other classes or which elements the attribute can be applied to. The following code fragment demonstrates how to use the AttributeUsageAttribute:
[AttributeUsage(AttributeTargets::All, Inherited = false, AllowMultiple = true)]
[AttributeUsage(AttributeTargets.All, Inherited = false, AllowMultiple = true)]
<AttributeUsage(AttributeTargets.All, Inherited:=False, AllowMultiple:=True)>
Public Class SomeClass
Inherits Attribute
'...
End Class
The AttributeUsageAttribute has three members that are important for the creation of custom attributes: AttributeTargets, Inherited, and AllowMultiple.
In the preceding example, AttributeTargets.All is specified, indicating that this attribute can be applied to all program elements. Alternatively, you can specify AttributeTargets.Class, indicating that your attribute can be applied only to a class, or AttributeTargets.Method, indicating that your attribute can be applied only to a method. All program elements can be marked for description by a custom attribute in this manner.
You can also pass multiple AttributeTargets values. The following code fragment specifies that a custom attribute can be applied to any class or method:
[AttributeUsage(AttributeTargets::Class | AttributeTargets::Method)]
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
<AttributeUsage(AttributeTargets.Class Or AttributeTargets.Method)>
Public Class SomeOtherClass
Inherits Attribute
'...
End Class
The AttributeUsageAttribute.Inherited property indicates whether your attribute can be inherited by classes that are derived from the classes to which your attribute is applied. This property takes either a true
(the default) or false
flag. In the following example, MyAttribute
has a default Inherited value of true
, while YourAttribute
has an Inherited value of false
:
// This defaults to Inherited = true.
public ref class MyAttribute : Attribute
{
//...
};
[AttributeUsage(AttributeTargets::Method, Inherited = false)]
public ref class YourAttribute : Attribute
{
//...
};
// This defaults to Inherited = true.
public class MyAttribute : Attribute
{
//...
}
[AttributeUsage(AttributeTargets.Method, Inherited = false)]
public class YourAttribute : Attribute
{
//...
}
' This defaults to Inherited = true.
Public Class MyAttribute
Inherits Attribute
'...
End Class
<AttributeUsage(AttributeTargets.Method, Inherited:=False)>
Public Class YourAttribute
Inherits Attribute
'...
End Class
The two attributes are then applied to a method in the base class MyClass
:
public ref class MyClass
{
public:
[MyAttribute]
[YourAttribute]
virtual void MyMethod()
{
//...
}
};
public class MyClass
{
[MyAttribute]
[YourAttribute]
public virtual void MyMethod()
{
//...
}
}
Public Class MeClass
<MyAttribute>
<YourAttribute>
Public Overridable Sub MyMethod()
'...
End Sub
End Class
Finally, the class YourClass
is inherited from the base class MyClass
. The method MyMethod
shows MyAttribute
but not YourAttribute
:
public ref class YourClass : MyClass
{
public:
// MyMethod will have MyAttribute but not YourAttribute.
virtual void MyMethod() override
{
//...
}
};
public class YourClass : MyClass
{
// MyMethod will have MyAttribute but not YourAttribute.
public override void MyMethod()
{
//...
}
}
Public Class YourClass
Inherits MeClass
' MyMethod will have MyAttribute but not YourAttribute.
Public Overrides Sub MyMethod()
'...
End Sub
End Class
The AttributeUsageAttribute.AllowMultiple property indicates whether multiple instances of your attribute can exist on an element. If set to true
, multiple instances are allowed. If set to false
(the default), only one instance is allowed.
In the following example, MyAttribute
has a default AllowMultiple value of false
, while YourAttribute
has a value of true
:
//This defaults to AllowMultiple = false.
public ref class MyAttribute : Attribute
{
};
[AttributeUsage(AttributeTargets::Method, AllowMultiple = true)]
public ref class YourAttribute : Attribute
{
};
//This defaults to AllowMultiple = false.
public class MyAttribute : Attribute
{
}
[AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
public class YourAttribute : Attribute
{
}
' This defaults to AllowMultiple = false.
Public Class MyAttribute
Inherits Attribute
End Class
<AttributeUsage(AttributeTargets.Method, AllowMultiple:=true)>
Public Class YourAttribute
Inherits Attribute
End Class
When multiple instances of these attributes are applied, MyAttribute
produces a compiler error. The following code example shows the valid use of YourAttribute
and the invalid use of MyAttribute
:
public ref class MyClass
{
public:
// This produces an error.
// Duplicates are not allowed.
[MyAttribute]
[MyAttribute]
void MyMethod()
{
//...
}
// This is valid.
[YourAttribute]
[YourAttribute]
void YourMethod()
{
//...
}
};
public class MyClass
{
// This produces an error.
// Duplicates are not allowed.
[MyAttribute]
[MyAttribute]
public void MyMethod()
{
//...
}
// This is valid.
[YourAttribute]
[YourAttribute]
public void YourMethod()
{
//...
}
}
Public Class MyClass
' This produces an error.
' Duplicates are not allowed.
<MyAttribute>
<MyAttribute>
Public Sub MyMethod()
'...
End Sub
' This is valid.
<YourAttribute>
<YourAttribute>
Public Sub YourMethod()
'...
End Sub
End Class
If both the AllowMultiple property and the Inherited property are set to true
, a class that's inherited from another class can inherit an attribute and have another instance of the same attribute applied in the same child class. If AllowMultiple is set to false
, the values of any attributes in the parent class will be overwritten by new instances of the same attribute in the child class.
After you apply the AttributeUsageAttribute, start defining the specifics of your attribute. The declaration of an attribute class looks similar to the declaration of a traditional class, as demonstrated by the following code:
[AttributeUsage(AttributeTargets::Method)]
public ref class MyAttribute : Attribute
{
// . . .
};
[AttributeUsage(AttributeTargets.Method)]
public class MyAttribute : Attribute
{
// . . .
}
<AttributeUsage(AttributeTargets.Method)>
Public Class MyAttribute
Inherits Attribute
' . . .
End Class
This attribute definition demonstrates the following points:
Attribute classes must be declared as public classes.
By convention, the name of the attribute class ends with the word Attribute. While not required, this convention is recommended for readability. When the attribute is applied, the inclusion of the word Attribute is optional.
All attribute classes must inherit directly or indirectly from the System.Attribute class.
In Microsoft Visual Basic, all custom attribute classes must have the System.AttributeUsageAttribute attribute.
Just like traditional classes, attributes are initialized with constructors. The following code fragment illustrates a typical attribute constructor. This public constructor takes a parameter and sets a member variable equal to its value.
MyAttribute(bool myvalue)
{
this->myvalue = myvalue;
}
public MyAttribute(bool myvalue)
{
this.myvalue = myvalue;
}
Public Sub New(myvalue As Boolean)
Me.myvalue = myvalue
End Sub
You can overload the constructor to accommodate different combinations of values. If you also define a property for your custom attribute class, you can use a combination of named and positional parameters when initializing the attribute. Typically, you define all required parameters as positional and all optional parameters as named. In this case, the attribute can't be initialized without the required parameter. All other parameters are optional.
Σημείωση
In Visual Basic, constructors for an attribute class shouldn't use a ParamArray
argument.
The following code example shows how an attribute that uses the previous constructor can be applied using optional and required parameters. It assumes that the attribute has one required Boolean value and one optional string property.
// One required (positional) and one optional (named) parameter are applied.
[MyAttribute(false, OptionalParameter = "optional data")]
public ref class SomeClass
{
//...
};
// One required (positional) parameter is applied.
[MyAttribute(false)]
public ref class SomeOtherClass
{
//...
};
// One required (positional) and one optional (named) parameter are applied.
[MyAttribute(false, OptionalParameter = "optional data")]
public class SomeClass
{
//...
}
// One required (positional) parameter is applied.
[MyAttribute(false)]
public class SomeOtherClass
{
//...
}
' One required (positional) and one optional (named) parameter are applied.
<MyAttribute(false, OptionalParameter:="optional data")>
Public Class SomeClass
'...
End Class
' One required (positional) parameter is applied.
<MyAttribute(false)>
Public Class SomeOtherClass
'...
End Class
If you want to define a named parameter or provide an easy way to return the values stored by your attribute, declare a property. Attribute properties should be declared as public entities with a description of the data type that will be returned. Define the variable that will hold the value of your property and associate it with the get
and set
methods. The following code example demonstrates how to implement a property in your attribute:
property bool MyProperty
{
bool get() {return this->myvalue;}
void set(bool value) {this->myvalue = value;}
}
public bool MyProperty
{
get {return this.myvalue;}
set {this.myvalue = value;}
}
Public Property MyProperty As Boolean
Get
Return Me.myvalue
End Get
Set
Me.myvalue = Value
End Set
End Property
This section incorporates the previous information and shows how to design an attribute that documents information about the author of a section of code. The attribute in this example stores the name and level of the programmer, and whether the code has been reviewed. It uses three private variables to store the actual values to save. Each variable is represented by a public property that gets and sets the values. Finally, the constructor is defined with two required parameters:
[AttributeUsage(AttributeTargets::All)]
public ref class DeveloperAttribute : Attribute
{
// Private fields.
private:
String^ name;
String^ level;
bool reviewed;
public:
// This constructor defines two required parameters: name and level.
DeveloperAttribute(String^ name, String^ level)
{
this->name = name;
this->level = level;
this->reviewed = false;
}
// Define Name property.
// This is a read-only attribute.
virtual property String^ Name
{
String^ get() {return name;}
}
// Define Level property.
// This is a read-only attribute.
virtual property String^ Level
{
String^ get() {return level;}
}
// Define Reviewed property.
// This is a read/write attribute.
virtual property bool Reviewed
{
bool get() {return reviewed;}
void set(bool value) {reviewed = value;}
}
};
[AttributeUsage(AttributeTargets.All)]
public class DeveloperAttribute : Attribute
{
// Private fields.
private string name;
private string level;
private bool reviewed;
// This constructor defines two required parameters: name and level.
public DeveloperAttribute(string name, string level)
{
this.name = name;
this.level = level;
this.reviewed = false;
}
// Define Name property.
// This is a read-only attribute.
public virtual string Name
{
get {return name;}
}
// Define Level property.
// This is a read-only attribute.
public virtual string Level
{
get {return level;}
}
// Define Reviewed property.
// This is a read/write attribute.
public virtual bool Reviewed
{
get {return reviewed;}
set {reviewed = value;}
}
}
<AttributeUsage(AttributeTargets.All)>
Public Class DeveloperAttribute
Inherits Attribute
' Private fields.
Private myname As String
Private mylevel As String
Private myreviewed As Boolean
' This constructor defines two required parameters: name and level.
Public Sub New(name As String, level As String)
Me.myname = name
Me.mylevel = level
Me.myreviewed = False
End Sub
' Define Name property.
' This is a read-only attribute.
Public Overridable ReadOnly Property Name() As String
Get
Return myname
End Get
End Property
' Define Level property.
' This is a read-only attribute.
Public Overridable ReadOnly Property Level() As String
Get
Return mylevel
End Get
End Property
' Define Reviewed property.
' This is a read/write attribute.
Public Overridable Property Reviewed() As Boolean
Get
Return myreviewed
End Get
Set
myreviewed = value
End Set
End Property
End Class
You can apply this attribute using the full name, DeveloperAttribute
, or using the abbreviated name, Developer
, in one of the following ways:
[Developer("Joan Smith", "1")]
-or-
[Developer("Joan Smith", "1", Reviewed = true)]
[Developer("Joan Smith", "1")]
-or-
[Developer("Joan Smith", "1", Reviewed = true)]
<Developer("Joan Smith", "1")>
-or-
<Developer("Joan Smith", "1", Reviewed := true)>
The first example shows the attribute applied with only the required named parameters. The second example shows the attribute applied with both the required and optional parameters.
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