Where do I put my extension method?

Question

A senior member here gave me this code:

public static string Truncate(this string value, int maxChars) {     return value.Length <= maxChars ? value : value.Substring(0, maxChars) + " .."; } 

He said to use it as an extension method. But where do I put this method? It looks like it adds something to .Net

Answer 1

Consider a class named StringExtensions like so:

static class StringExtensions {     public static string Truncate(this string value, int maxChars)     {         return value.Length <= maxChars ?                 value :                 value.Substring(0, maxChars) + " ..";     } } 

Be sure that whatever namespace you put this class in, you include a using declaration for that namespace.

Thus, for a full example:

StringExtensions.cs:

namespace My.Extensions {     static class StringExtensions     {         public static string Truncate(this string value, int maxChars)         {                    return value.Length <= maxChars ?                    value :                    value.Substring(0, maxChars) + " ..";         }     } } 

Program.cs:

using System; using My.Extensions;  namespace My.Program {     static class Program     {         static void Main(string[] args)         {             string s = "Hello, World";             string t = s.Truncate(5);             Console.WriteLine(s);             Console.WriteLine(t);         }     } } 

By the way, you are not adding it to .NET. You are not even adding a new method to the class String. Rather, it's a compiler trick that makes static methods living in static classes with their first parameter declared as this *TypeName* *valueParameter* where *TypeName* is the name of a type, and *valueParameter* is the name of the parameter can be made to appear as an instance method on instances of the type with type name *TypeName*. That is

string t = s.Truncate(5); 

is translated by the compiler into

string t = StringExtensions.Truncate(s, 5); 

Answer 2

Put it in a static class, and use using on its namespace.

e.g.

namespace Foo {     static class Extensions     {         public static string Truncate(this string value, int maxChars)         {             return value.Length <= maxChars ?                 value : value.Substring(0, maxChars) + " ..";         }     } } 

And then in a different file:

using Foo;  //Don't forget this!  class Tester {     static void Test()     {         Console.WriteLine("123456".Truncate(3));     } }