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);