Attributes in C#

Attributes are used in C# to convey declarative information or metadata about various code elements such as methods, assemblies, properties, types, etc. Attributes are added to the code by using a declarative tag that is placed using square brackets ([ ]) on top of the required code element. There are two types of Attributes implementations provided by the .NET Framework are:

Properties of Attributes

• Attributes can have arguments just like methods, properties, etc. can have arguments.
• Attributes can have zero or more parameters.
• Different code elements such as methods, assemblies, properties, types, etc. can have one or multiple attributes.
• Reflection can be used to obtain the metadata of the program by accessing the attributes at run-time.
• Attributes are generally derived from the System. Attribute Class.

Predefined Attributes

Predefined attributes are those attributes that are a part of the .NET Framework Class Library and are supported by the C# compiler for a specific purpose. Some of the predefined attributes are derived from the System. Attribute base classes are given as follows: 

Attribute	Description
AttributeUsageAttribute	This attribute specifies the usage of a different attribute.
CLSCompliantAttribute	This attribute shows if a particular code element complies with the Common Language Specification.
ContextStaticAttribute	This attribute designates that a static field should not be shared between contexts.
FlagsAttribute	The FlagsAttribute specifies that an enumeration can be used as a set of flags. This is most commonly used with bitwise operators.
LoaderOptimizationAttribute	This attribute sets the optimization policy for the default loader in the main method.
NonSerializedAttribute	This attribute signifies that the field of the serializable class should not be serialized.
ObsoleteAttribute	This attribute marks the code elements that are obsolete i.e. not in use anymore.
SerializableAttribute	This attribute signifies that the field of the serializable class can be serialized.
ThreadStaticAttribute	This attribute indicates that there is a unique static field value for each thread.
DllImportAttribute	This attribute indicates that the method is a static entry point as shown by the unmanaged DLL.

Let’s discuss some of the predefined attributes:

CLSCompliantAttribute

This attribute shows if a particular code element complies with the Common Language Specification. If a particular code element complies with the Common Language Specification. If it doesn’t, then a warning message is issued by the compiler. 

Example 1: Here, it will not give any warning message, and the code compiles successfully. 

csharp

// C# program to demonstrate CLSCompliantAttribute
using System;

// CLSCompliantAttribute applied to entire assembly
[assembly:CLSCompliant(true)]

    public class GFG {

    // Main Method
    public static void Main(string[] args)
    {
        Console.WriteLine("GeeksForGeeks");
    }
}

Output:

GeeksForGeeks

Example 2: This code will give a warning message by the compiler. 

csharp

// C# program to demonstrate CLSCompliantAttribute
// giving a warning message
using System;

// CLSCompliantAttribute applied to entire assembly
[assembly:CLSCompliant(true)]

    public class GFG {
    public uint z;
}

class GFG2 {

// Main Method
public static void Main(string[] args)
{
    Console.WriteLine("GeeksForGeeks");
}

}

Warning:

prog.cs(9,14): warning CS3003: Type of `GFG.z’ is not CLS-compliant

FlagsAttribute

The FlagsAttribute specifies that an enumeration can be used as a set of flags. This is most commonly used with bitwise operators. 

Example 3:

csharp

// C# program to demonstrate FlagsAttribute
using System;

class GFG {

    // Enum defined without FlagsAttribute.
    enum Colours { Red = 1,
                   Blue = 2,
                   Pink = 4,
                   Green = 8
    }

    // Enum defined with FlagsAttribute.
    [Flags] enum ColoursFlags { Red = 1,
                                Blue = 2,
                                Pink = 4,
                                Green = 8
    }

    // Main Method
    public static void Main(string[] args)
    {
        Console.WriteLine((Colours.Red | Colours.Blue).ToString());
        Console.WriteLine((ColoursFlags.Red | ColoursFlags.Blue).ToString());
    }
}

Output:

3
Red,Blue

ObsoleteAttribute

The ObsoleteAttribute marks the code elements that are obsolete i.e. not in use anymore. Calling these obsolete code elements results in a compiler error. 

Example 4:

csharp

// C# program to demonstrate ObsoleteAttribute
using System;

class GFG {

    // The method1() is marked as obsolete
    [Obsolete("method1 is obsolete", true)] static void method1()
    {
        Console.WriteLine("This is method1");
    }

    static void method2()
    {
        Console.WriteLine("This is method2");
    }

    public static void Main(string[] args)
    {
        method1(); // Compiler error as method1() is obsolete
        method2();
    }
}

Compile Errors:

prog.cs(18,3): error CS0619: `GFG.method1()’ is obsolete: `method1 is obsolete’

Custom Attributes

Custom attributes can be created in C# for attaching declarative information to methods, assemblies, properties, types, etc. in any way required. This increases the extensibility of the .NET framework. Steps for creating Custom Attributes:

• Define a custom attribute class that is derived from System. Attribute class.
• The custom attribute class name should have the suffix Attribute.
• Use the attribute AttributeUsage to specify the usage of the custom attribute class created.
• Create the constructor and the accessible properties of the custom attribute class.