c# - 'string' does not contain a definition for 'isNullorWhiteSpace' -

i'm not sure how incorporate own method code using isnullorwhitespace, framework isn't 4.0. i've had , suggested using isnullorwhitespace, not preferred method display:

2/20/2014 7:33:10 am, measured velocity: 0.2225

i can't seem find equivalent code work.

 using system;     using system.collections.generic;     using system.text;     using system.io;     using system.collections;      namespace conapp     {         class program         {             public static class stringextensions             {                 public static bool isnullorwhitespace(string value)                 {                     if (value != null)                     {                         (int = 0; < value.length; i++)                         {                             if (!char.iswhitespace(value[i]))                             {                                 return false;                             }                         }                     }                     return true;                 }             }              static void main(string[] args)             {                 string line;                 try                 {                     using (streamwriter sw = new streamwriter("c:\\writetest\\writetest.txt"))                     {                         string mydirpath = "c:\\chat\\";                          string[] txtfilelist = directory.getfiles(mydirpath, "*.txt");                          foreach (string txtname in txtfilelist)                         {                             system.io.streamreader sr = new system.io.streamreader(txtname);                              while ((line = sr.readline()) != null)                             {                                 string spart = ".prt";                                 string sam = " am";                                 string spm = " pm";                                 string sresult = "test result: ";                                 string svelocity = "measured velocity: ";                                 string part = string.empty;                                 string date = string.empty;                                 string result = string.empty;                                 string velocity = string.empty;                                 // sw.writeline(line);                                      if (line.contains(sam) || line.contains(spm))                                     {                                         date = line;                                     }                                      if (line.contains(spart))                                       {                                            part = line;                                       }                                      if (line.contains(sresult))                                       {                                           result = line;                                       }                                      if (line.contains(svelocity))                                       {                                           velocity = line;                                       }                                    if (!string.isnullorwhitespace(date) && !string.isnullorwhitespace(velocity))                                   {                                bool isnullorwhitespace = "foo bar".isnullorwhitespace(); //doesnt work here                                        int = 2;                                       string[] x = new string[i];                                       x[0] = date;                                       x[1] = velocity;                                       sw.writeline(x[0] + "," + x[1]);                                   }                                }                          }                     }                 }                 catch                 {                  }             }          }     } 

firstly, stringextensions needs top level class, cannot inside class.
secondly, need turn method extension method adding this keyword first parameter:

public static bool isnullorwhitespace(this string value) 

so becomes:

public static class stringextensions {     public static bool isnullorwhitespace(this string value)     {         if (value != null) {             (int = 0; < value.length; i++) {                 if (!char.iswhitespace(value[i])) {                     return false;                 }             }         }         return true;     } }  class program {     ... }