c# - BouncyCastle IPasswordFinder memory management -

i'm writing small wrapper bouncycastle basic crypto tasks. have class reads pem file, , such supports password protected pem files. support passwords, overrode ipasswordfinder , pass pemreader. works fine, noticed 1 strange thing passwordfinder when analyzing code coverage report. class takes securestring, temporarily marshals bouncycastle can read it, , disposes of immediately. when dispose code gets called, however, buffer freed. wondering if bouncycastle disposing of buffer itself, or if else going on. code:

internal class securepasswordstore : ipasswordfinder, idisposable {      private readonly securestring password;     private char[] _chars;      public securepasswordstore(securestring password) {         this.password = password;     }      public unsafe char[] getpassword() {         intptr ptr = intptr.zero;         try {             _chars = new char[password.length];             ptr = marshal.securestringtocotaskmemunicode(password);             marshal.copy(ptr, _chars, 0, password.length);             return _chars;         } {             if (ptr != intptr.zero) {                 marshal.zerofreecotaskmemunicode(ptr);             }         }     }      #region idisposable support     private bool disposedvalue = false; // detect redundant calls       protected virtual void dispose(bool disposing) {         if (!disposedvalue) {             if (disposing) {                 (var = 0; < _chars?.length; i++) {                     _chars[i] = default(char);                 }                 _chars = null;             }             disposedvalue = true;         }     }      ~securepasswordstore() {         dispose(false);     }      // code added correctly implement disposable pattern.     public void dispose() {         dispose(true);         gc.suppressfinalize(this);     }     #endregion } 

so essentially, getpassword gets called pemreader, securestring marshalled _chars array. when password store disposed, _chars array should nulled out , disposed of. however, time dispose method, _chars null again. can shed light on this?