Common Attributes (C# and Visual Basic)

This topic describes the attributes that are most commonly used in C# and Visual Basic programs.

• Global Attributes

• Obsolete Attribute

• Conditional Attribute

• Caller Info Attributes

• Visual Basic Attributes

Global Attributes

Most attributes are applied to specific language elements such as classes or methods; however, some attributes are global—they apply to an entire assembly or module. For example, the AssemblyVersionAttribute attribute can be used to embed version information into an assembly, like this:

[assembly: AssemblyVersion("1.0.0.0")]

<Assembly: AssemblyVersion("1.0.0.0")>

Global attributes appear in the source code after any top-level using directives (Imports in Visual Basic) and before any type, module, or namespace declarations. Global attributes can appear in multiple source files, but the files must be compiled in a single compilation pass. For Visual Basic projects, global attributes are generally put in the AssemblyInfo.vb file that is created automatically with Visual Basic projects. In C# projects, they are put in the AssemblyInfo.cs file.

Assembly attributes are values that provide information about an assembly. They fall into the following categories:

• Assembly identity attributes

• Informational attributes

• Assembly manifest attributes

• Strong name attributes

Assembly Identity Attributes

Three attributes (with a strong name, if applicable) determine the identity of an assembly: name, version, and culture. These attributes form the full name of the assembly and are required when you reference it in code. You can set an assembly's version and culture using attributes. However, the name value is set by the compiler, the Visual Studio IDE in the Assembly Information Dialog Box, or the Assembly Linker (Al.exe) when the assembly is created, based on the file that contains the assembly manifest. The AssemblyFlagsAttribute attribute specifies whether multiple copies of the assembly can coexist.

The following table shows the identity attributes.

Informational Attributes

You can use informational attributes to provide additional company or product information for an assembly. The following table shows the informational attributes defined in the System.Reflection namespace.

Assembly Manifest Attributes

You can use assembly manifest attributes to provide information in the assembly manifest. This includes title, description, default alias, and configuration. The following table shows the assembly manifest attributes defined in the System.Reflection namespace.

Strong Name Attributes

In previous versions of Visual Studio, signing assemblies by using strong names was performed with these assembly-level attributes:

• AssemblyKeyFileAttribute

• AssemblyKeyNameAttribute

• AssemblyDelaySignAttribute

This is still supported, but the preferred way to sign assemblies is to use the Signing Page in the Project Designer. See Signing Page, Project Designer and How to: Sign an Assembly (Visual Studio) for more information.

Obsolete Attribute

The Obsolete attribute marks a program entity as one that is no longer recommended for use. Each use of an entity marked obsolete will subsequently generate a warning or an error, depending on how the attribute is configured. For example:

    <System.Obsolete("use class B")> 
    Class A
        Sub Method()
        End Sub 
    End Class 

    Class B
        <System.Obsolete("use NewMethod", True)> 
        Sub OldMethod()
        End Sub 

        Sub NewMethod()
        End Sub 
    End Class

[System.Obsolete("use class B")]
class A
{
    public void Method() { }
}
class B
{
    [System.Obsolete("use NewMethod", true)]
    public void OldMethod() { }
    public void NewMethod() { }
}

In this example the Obsolete attribute is applied to class A and to method B.OldMethod. Because the second argument of the attribute constructor applied to B.OldMethod is set to true, this method will cause a compiler error, whereas using class A will just produce a warning. Calling B.NewMethod, however, produces no warning or error.

The string provided as the first argument to attribute constructor will be displayed as part of the warning or error. For example, when you use it with the previous definitions, the following code generates two warnings and one error:

' Generates 2 warnings: 
' Dim a As New A 
' Generate no errors or warnings: 

Dim b As New B
b.NewMethod()

' Generates an error, terminating compilation: 
' b.OldMethod()

// Generates 2 warnings: 
// A a = new A(); 

// Generate no errors or warnings:
B b = new B();
b.NewMethod();

// Generates an error, terminating compilation: 
// b.OldMethod();

Two warnings for class A are generated: one for the declaration of the class reference, and one for the class constructor.

The Obsolete attribute can be used without arguments, but including an explanation of why the item is obsolete and what to use instead is recommended.

The Obsolete attribute is a single-use attribute and can be applied to any entity that allows attributes. Obsolete is an alias for ObsoleteAttribute.

Conditional Attribute

The Conditional attribute makes the execution of a method dependent on a preprocessing identifier. The Conditional attribute is an alias for ConditionalAttribute, and can be applied to a method or an attribute class.

In this example, Conditional is applied to a method to enable or disable the display of program-specific diagnostic information:

#Const TRACE_ON = True 
Imports System
Imports System.Diagnostics
Module TestConditionalAttribute
    Public Class Trace
        <Conditional("TRACE_ON")> 
        Public Shared Sub Msg(ByVal msg As String)
            Console.WriteLine(msg)
        End Sub 

    End Class 

    Sub Main()
        Trace.Msg("Now in Main...")
        Console.WriteLine("Done.")
    End Sub 
End Module

#define TRACE_ON
using System;
using System.Diagnostics;

public class Trace
{
    [Conditional("TRACE_ON")]
    public static void Msg(string msg)
    {
        Console.WriteLine(msg);
    }
}

public class ProgramClass
{
    static void Main()
    {
        Trace.Msg("Now in Main...");
        Console.WriteLine("Done.");
    }
}

If the TRACE_ON identifier is not defined, no trace output will be displayed.

The Conditional attribute is often used with the DEBUG identifier to enable trace and logging features for debug builds but not in release builds, like this:

<Conditional("DEBUG")> 
Shared Sub DebugMethod()

End Sub

[Conditional("DEBUG")]
static void DebugMethod()
{
}

When a method marked as conditional is called, the presence or absence of the specified preprocessing symbol determines whether the call is included or omitted. If the symbol is defined, the call is included; otherwise, the call is omitted. Using Conditional is a cleaner, more elegant, and less error-prone alternative to enclosing methods inside #if…#endif blocks, like this:

#If DEBUG Then 
    Sub ConditionalMethod()
    End Sub
#End If

#if DEBUG
    void ConditionalMethod()
    {
    }
#endif

A conditional method must be a method in a class or struct declaration and must not have a return value.

Using Multiple Identifiers

If a method has multiple Conditional attributes, a call to the method is included if at least one of the conditional symbols is defined (in other words, the symbols are logically linked together by using the OR operator). In this example, the presence of either A or B will result in a method call:

<Conditional("A"), Conditional("B")> 
Shared Sub DoIfAorB()

End Sub

[Conditional("A"), Conditional("B")]
static void DoIfAorB()
{
    // ...
}

To achieve the effect of logically linking symbols by using the AND operator, you can define serial conditional methods. For example, the second method below will execute only if both A and B are defined:

<Conditional("A")> 
Shared Sub DoIfA()
    DoIfAandB()
End Sub

<Conditional("B")> 
Shared Sub DoIfAandB()
    ' Code to execute when both A and B are defined... 
End Sub

[Conditional("A")]
static void DoIfA()
{
    DoIfAandB();
}

[Conditional("B")]
static void DoIfAandB()
{
    // Code to execute when both A and B are defined...
}

Using Conditional with Attribute Classes

The Conditional attribute can also be applied to an attribute class definition. In this example, the custom attribute Documentation will only add information to the metadata if DEBUG is defined.

<Conditional("DEBUG")> 
Public Class Documentation
    Inherits System.Attribute
    Private text As String 
    Sub New(ByVal doc_text As String)
        text = doc_text
    End Sub 
End Class 

Class SampleClass
    ' This attribute will only be included if DEBUG is defined.
    <Documentation("This method displays an integer.")> 
    Shared Sub DoWork(ByVal i As Integer)
        System.Console.WriteLine(i)
    End Sub 
End Class

[Conditional("DEBUG")]
public class Documentation : System.Attribute
{
    string text;

    public Documentation(string text)
    {
        this.text = text;
    }
}

class SampleClass
{
    // This attribute will only be included if DEBUG is defined.
    [Documentation("This method displays an integer.")]
    static void DoWork(int i)
    {
        System.Console.WriteLine(i.ToString());
    }
}

Caller Info Attributes

By using Caller Info attributes, you can obtain information about the caller to a method. You can obtain the file path of the source code, the line number in the source code, and the member name of the caller.

To obtain member caller information, you use attributes that are applied to optional parameters. Each optional parameter specifies a default value. The following table lists the Caller Info attributes that are defined in the System.Runtime.CompilerServices namespace:

For more information about the Caller Info attributes, see Caller Information (C# and Visual Basic).

Visual Basic Attributes

The following table lists the attributes that are specific to Visual Basic.

COMClassAttribute

Use COMClassAttribute to simplify the process of creating COM components from Visual Basic. COM objects are considerably different from .NET Framework assemblies, and without COMClassAttribute, you need to follow a number of steps to generate a COM object from Visual Basic. For classes marked with COMClassAttribute, the compiler performs many of these steps automatically.

HideModuleNameAttribute

Use HideModuleNameAttribute to allow module members to be accessed by using only the qualification needed for the module.

VBFixedStringAttribute

Use VBFixedStringAttribute to force Visual Basic to create a fixed-length string. Strings are of variable length by default, and this attribute is useful when storing strings to files. The following code demonstrates this:

Structure Worker
    ' The runtime uses VBFixedString to determine  
    ' if the field should be written out as a fixed size.
    <VBFixedString(10)> Public LastName As String
    <VBFixedString(7)> Public Title As String
    <VBFixedString(2)> Public Rank As String 
End Structure

VBFixedArrayAttribute

Use VBFixedArrayAttribute to declare arrays that are fixed in size. Like Visual Basic strings, arrays are of variable length by default. This attribute is useful when serializing or writing data to files.

See Also

Reference

Reflection (C# and Visual Basic)

Accessing Attributes by Using Reflection (C# and Visual Basic)

System.Reflection

Attribute

Concepts

C# Programming Guide

Other Resources

Visual Basic Programming Guide

Extending Metadata Using Attributes