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Extension methods add functionality to existing libraries.You might use extension methods without even knowing it.For example, when you use code completion in an IDE,it suggests extension methods alongside regular methods.
As the example shows,you can invoke extension methods implicitly even if you import using a prefix.The only time you need to use the prefix isto avoid a name conflict when invoking an extension explicitly.
The members of an extension can be methods, getters, setters, or operators.Extensions can also have static fields and static helper methods.To access static members outside the extension declaration, invoke them through the declaration name like class variables and methods.
Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are static methods, but they're called as if they were instance methods on the extended type. For client code written in C#, F# and Visual Basic, there's no apparent difference between calling an extension method and the methods defined in a type.
The most common extension methods are the LINQ standard query operators that add query functionality to the existing System.Collections.IEnumerable and System.Collections.Generic.IEnumerable types. To use the standard query operators, first bring them into scope with a using System.Linq directive. Then any type that implements IEnumerable appears to have instance methods such as GroupBy, OrderBy, Average, and so on. You can see these additional methods in IntelliSense statement completion when you type "dot" after an instance of an IEnumerable type such as List or Array.
The following example shows how to call the standard query operator OrderBy method on an array of integers. The expression in parentheses is a lambda expression. Many standard query operators take lambda expressions as parameters, but this isn't a requirement for extension methods. For more information, see Lambda Expressions.
You invoke the extension method in your code with instance method syntax. The intermediate language (IL) generated by the compiler translates your code into a call on the static method. The principle of encapsulation is not really being violated. Extension methods cannot access private variables in the type they are extending.
In general, you'll probably be calling extension methods far more often than implementing your own. Because extension methods are called by using instance method syntax, no special knowledge is required to use them from client code. To enable extension methods for a particular type, just add a using directive for the namespace in which the methods are defined. For example, to use the standard query operators, add this using directive to your code:
You can use extension methods to extend a class or interface, but not to override them. An extension method with the same name and signature as an interface or class method will never be called. At compile time, extension methods always have lower priority than instance methods defined in the type itself. In other words, if a type has a method named Process(int i), and you have an extension method with the same signature, the compiler will always bind to the instance method. When the compiler encounters a method invocation, it first looks for a match in the type's instance methods. If no match is found, it will search for any extension methods that are defined for the type, and bind to the first extension method that it finds.
The following example demonstrates the rules that the C# compiler follows in determining whether to bind a method call to an instance method on the type, or to an extension method. The static class Extensions contains extension methods defined for any type that implements IMyInterface. Classes A, B, and C all implement the interface.
In the past, it was common to create "Collection Classes" that implemented the System.Collections.Generic.IEnumerable interface for a given type and contained functionality that acted on collections of that type. While there's nothing wrong with creating this type of collection object, the same functionality can be achieved by using an extension on the System.Collections.Generic.IEnumerable. Extensions have the advantage of allowing the functionality to be called from any collection such as an System.Array or System.Collections.Generic.List that implements System.Collections.Generic.IEnumerable on that type. An example of this using an Array of Int32 can be found earlier in this article.
Rather than creating new objects when reusable functionality needs to be created, we can often extend an existing type, such as a .NET or CLR type. As an example, if we don't use extension methods, we might create an Engine or Query class to do the work of executing a query on a SQL Server that may be called from multiple places in our code. However we can instead extend the System.Data.SqlClient.SqlConnection class using extension methods to perform that query from anywhere we have a connection to a SQL Server. Other examples might be to add common functionality to the System.String class, extend the data processing capabilities of the System.IO.File and System.IO.Stream objects, and System.Exception objects for specific error handling functionality. These types of use-cases are limited only by your imagination and good sense.
Extending predefined types can be difficult with struct types because they're passed by value to methods. That means any changes to the struct are made to a copy of the struct. Those changes aren't visible once the extension method exits. You can add the ref modifier to the first argument making it a ref extension method. The ref keyword can appear before or after the this keyword without any semantic differences. Adding the ref modifier indicates that the first argument is passed by reference. This enables you to write extension methods that change the state of the struct being extended (note that private members are not accessible). Only value types or generic types constrained to struct (see struct constraint for more information) are allowed as the first parameter of a ref extension method. The following example shows how to use a ref extension method to directly modify a built-in type without the need to reassign the result or pass it through a function with the ref keyword:
While it's still considered preferable to add functionality by modifying an object's code or deriving a new type whenever it's reasonable and possible to do so, extension methods have become a crucial option for creating reusable functionality throughout the .NET ecosystem. For those occasions when the original source isn't under your control, when a derived object is inappropriate or impossible, or when the functionality shouldn't be exposed beyond its applicable scope, Extension methods are an excellent choice.
For a class library that you implemented, you shouldn't use extension methods to avoid incrementing the version number of an assembly. If you want to add significant functionality to a library for which you own the source code, follow the .NET guidelines for assembly versioning. For more information, see Assembly Versioning.
This extension will continue to apply the guidance previously issued for employees hired on or after April 1, 2021, and work exclusively in a remote setting due to COVID-19-related precautions. Those employees are temporarily exempt from the physical inspection requirements associated with the Employment Eligibility Verification (Form I-9) until they undertake non-remote employment on a regular, consistent, or predictable basis, or the extension of the flexibilities related to such requirements is terminated, whichever is earlier.
If employees hired on or after April 1, 2021 work exclusively in a remote setting due to COVID-19 related precautions, they are temporarily exempt from the physical inspection requirements associated with the Employment Eligibility Verification (Form I-9) under Section 274A of the INA until they undertake non-remote employment on a regular, consistent, or predictable basis, or the extension of the flexibilities related to such requirements is terminated, whichever is earlier.
Going forward DHS will continue to monitor the ongoing national emergency and provide updated guidance as needed. Employers are required to monitor the DHS and ICE websites for additional updates regarding when the extensions will be terminated, and normal operations will resume.
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