c# added extension method but cannot use it -

i have add extension method arraysegment follows, when using

     var lines = textcontrol.lines;      arraysegment<string> myarrsegofrichtextcontrol =                                new arraysegment<string>(lines,0,2); 

i cannot find visual studio prompt getsegment method after type myarrsegofrichtextcontrol. how can call method of extension? thanks.

namespace arraysegmentextension {     class arraysegmentextension      {         #region arraysegment related methods          public static arraysegment<t> getsegment<t>(this t[] array, int from, int count)         {             return new arraysegment<t>(array, from, count);         }          public static arraysegment<t> getsegment<t>(this t[] array, int from)         {             return getsegment(array, from, array.length - from);         }          public static arraysegment<t> getsegment<t>(this t[] array)         {             return new arraysegment<t>(array);         }          public static ienumerable<t> asenumerable<t>(this arraysegment<t> arraysegment)         {             return arraysegment.array.skip(arraysegment.offset).take(arraysegment.count);         }          public static t[] toarray<t>(this arraysegment<t> arraysegment)         {             t[] array = new t[arraysegment.count];             array.copy(arraysegment.array, arraysegment.offset, array, 0, arraysegment.count);             return array;         }          #endregion     } } 

extension methods must defined on static class. official documentation:

1. define static class contain extension method.

try defining class this:

public static class arraysegmentextension {     ... }