Define accessors for attribute arguments

Cause

In its constructor, an attribute defines arguments that do not have corresponding properties.

Rule Description

Attributes can define mandatory arguments that must be specified when applying the attribute to a target. These are sometimes called positional arguments because they are supplied to attribute constructors as positional parameters. For every mandatory argument, the attribute should also provide a corresponding read-only property so that the value of the argument can be retrieved at execution time. This rule checks to see that for each constructor parameter, you have defined the corresponding property.

Attributes can also define optional arguments, called named arguments. These arguments are supplied to attribute constructors by name and should have a corresponding read/write property.

For mandatory and optional arguments, the corresponding properties and constructor parameters should use the same name but different casing. Properties use Pascal casing, and parameters use camel casing.

How to Fix Violations

To fix a violation of this rule, add a read-only property for each constructor parameter that does not have one.

When to Exclude Warnings

Exclude a warning from this rule if you do not want the value of the mandatory argument to be retrievable.

Example

The following example shows two attributes that define a mandatory (positional) parameter. The first implementation of the attribute is incorrectly defined. The second implementation is correct.

Imports System

Namespace DesignLibrary

' Violates rule: DefineAccessorsForAttributeArguments.
<AttributeUsage(AttributeTargets.All)>  _
NotInheritable Public Class BadCustomAttribute
    Inherits Attribute
    Private data As String
    
    ' Missing the property that corresponds to 
    ' the someStringData parameter.
    Public Sub New(someStringData As String)
        data = someStringData
    End Sub 'New
End Class 'BadCustomAttribute

' Satisfies rule: Attributes should have accessors for all arguments.
<AttributeUsage(AttributeTargets.All)>  _
NotInheritable Public Class GoodCustomAttribute
    Inherits Attribute
    Private data As String
    
    Public Sub New(someStringData As String)
        data = someStringData
    End Sub 'New

    'The constructor parameter and property
    'name are the same except for case.
    
    Public ReadOnly Property SomeStringData() As String
        Get
            Return data
        End Get
    End Property
End Class 

End Namespace

using System;

namespace DesignLibrary
{
// Violates rule: DefineAccessorsForAttributeArguments.

   [AttributeUsage(AttributeTargets.All)]
   public sealed class BadCustomAttribute :Attribute 
   {
      string data;

      // Missing the property that corresponds to 
      // the someStringData parameter.

      public BadCustomAttribute(string someStringData)
      {
         data = someStringData;
      }
   }

// Satisfies rule: Attributes should have accessors for all arguments.

   [AttributeUsage(AttributeTargets.All)]
   public sealed class GoodCustomAttribute :Attribute 
   {
      string data;

      public GoodCustomAttribute(string someStringData)
      {
         data = someStringData;
      }
      //The constructor parameter and property
      //name are the same except for case.

      public string SomeStringData
      {
         get 
         {
            return data;
         }
      }
   }
}

Positional and named arguments make it clear to consumers of your library which arguments are mandatory for the attribute and which arguments are optional.

The following example shows an implementation of an attribute with both positional and named arguments.

The following example illustrates how to apply the above custom attribute to two properties.

Related Rules

Avoid unsealed attributes

See Also

Reference

Attribute Usage Guidelines