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:
- Predefined Attributes
- Custom Attributes
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.
1. 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 that are derived from the System.Attribute base class are given as follows:
|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 indicates if a static field value is unique for the specified context.|
|FlagsAttribute||This attribute indicates if a static field value is unique for the specified context.|
|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:
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 code compiles successfully.
Example 2: This code will give a warning message by the compiler.
prog.cs(9,14): warning CS3003: Type of `GFG.z’ is not CLS-compliant
The FlagsAttribute specifies that an enumeration can be used as a set of flags. This is most commonly used with bitwise operators.
3 Red, Blue
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.
prog.cs(18,3): error CS0619: `GFG.method1()’ is obsolete: `method1 is obsolete’
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.
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