How do I extend a class with c extension methods

Question

Can extension methods be applied to the class   For example  extend DateTime to include a Tomorrow   method that could be invoked like   DateTime Tomorrow      I know I can use   static DateTime Tomorrow this Datetime value              Or  public static MyClass     public static Tomorrow                 for a similar result  but how can I extend DateTime so that I could invoke DateTime Tomorrow

User · Answer

The closest I can get to the answer is by adding an extension method into a System.Type object. Not pretty, but still interesting.

public static class Foo
{
    public static void Bar()
    {
        var now = DateTime.Now;
        var tomorrow = typeof(DateTime).Tomorrow();
    }

    public static DateTime Tomorrow(this System.Type type)
    {
        if (type == typeof(DateTime)) {
            return DateTime.Now.AddDays(1);
        } else {
            throw new InvalidOperationException();
        }
    }
}

Otherwise, IMO Andrew and ShuggyCoUk has a better implementation.

User · Answer

Unfortunately  you can t do that  I believe it would be useful  though  It is more natural to type   DateTime Tomorrow   than   DateTimeUtil Tomorrow   With a Util class  you have to check for the existence of a static method in two different classes  instead of one

User · Answer

Extension methods are syntactic sugar for making static methods whose first parameter is an instance of type T look as if they were an instance method on T   As such the benefit is largely lost where you to make  static extension methods  since they would serve to confuse the reader of the code even more than an extension method  since they appear to be fully qualified but are not actually defined in that class  for no syntactical gain  being able to chain calls in a fluent style within Linq for example    Since you would have to bring the extensions into scope with a using anyway I would argue that it is simpler and safer to create    public static class DateTimeUtils       public static DateTime Tomorrow   get               And then use this in your code via   WriteLine   0    DateTimeUtils Tomorrow

User · Answer

You cannot add methods to an existing type unless the existing type is marked as partial  you can only add methods that appear to be a member of the existing type through extension methods   Since this is the case you cannot add static methods to the type itself since extension methods use instances of that type   There is nothing stopping you from creating your own static helper method like this   static class DateTimeHelper       public static DateTime Tomorrow               get   return DateTime Now AddDays 1               Which you would use like this   DateTime tomorrow   DateTimeHelper Tomorrow

User · Answer

I was looking for something similar - a list of constraints on classes that provide Extension Methods  Seems tough to find a concise list so here goes    You can t have any private or protected anything - fields  methods  etc  It must be a static class  as in public static class     Only methods can be in the class  and they must all be public static  You can t have conventional static methods - ones that don t include a this argument aren t allowed  All methods must begin   public static ReturnType MethodName this ClassName  this         So the first argument is always the this reference   There is an implicit problem this creates - if you add methods that require a lock of any sort  you can t really provide it at the class level  Typically you d provide a private instance-level lock  but it s not possible to add any private fields  leaving you with some very awkward options  like providing it as a public static on some outside class  etc  Gets dicey  Signs the C  language had kind of a bad turn in the design for these   The workaround is to use your Extension Method class as just a Facade to a regular class  and all the static methods in your Extension class just call the real class  probably using a Singleton

User · Answer

We have improved our answer with detail explanation Now it s more easy to understand about extension method  Extension method  It is a mechanism through which we can extend the behavior of existing class without using the sub classing or modifying or recompiling the original class or struct     We can extend our custom classes   net framework classes etc   Extension method is actually a special kind of static method that is defined in the static class   As DateTime class is already taken above and hence we have not taken this class for the explanation   Below is the example    This is a existing Calculator class which have only one method Add   public class Calculator        public double Add double num1  double num2                return num1   num2               Below is the extension class which have one extension method    public static class Extension          It is extension method and it s first parameter is a calculator class It s behavior is going to extend       public static double Division this Calculator cal  double num1 double num2          return num1   num2                 We have tested the extension method below          class Program       static void Main string   args                Calculator cal   new Calculator            double add cal Add 10  10              It is a extension method in Calculator class          double add cal Division 100  10

User · Answer

I would do the same as Kumu  namespace ExtensionMethods       public static class MyExtensionMethods               public static DateTime Tomorrow this DateTime date                       return date AddDays 1                           but call it like this new DateTime   Tomorrow     Think it makes more seens than DateTime Now Tomorrow

User · Answer

Use an extension method   Ex   namespace ExtensionMethods       public static class MyExtensionMethods               public static DateTime Tomorrow this DateTime date                        return date AddDays 1                           Usage   DateTime Now Tomorrow      or  AnyObjectOfTypeDateTime Tomorrow

User · Answer

They provide the capability to extend existing types by adding new methods with no modifications necessary to the type  Calling methods from objects of the extended type within an application using instance method syntax is known as       extending       methods  Extension methods are not instance members on the type  The key point to remember is that extension methods  defined as static methods  are in scope only when the namespace is explicitly imported into your application source code via the using directive  Even though extension methods are defined as static methods  they are still called using instance syntax   Check the full example here http   www dotnetreaders com articles Extension methods in C-sharp net Methods in C -sharp 201  Example   class Extension               static void Main string   args                        string s    sudhakar               Console WriteLine s GetWordCount                 Console ReadLine                         public static class MyMathExtension                public static int GetWordCount this System String mystring                        return mystring Length

[c#] How do I extend a class with c# extension methods?

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