Load an external movie at runtime

Hi, I want to achieve a demo that user can select video file via a file dialog and play it. Now I have met a problem that the video can not be loaded saying " MoviePlayback::LoadMovieData got NULL! UnityEngine.WWW:get_movie() ". But after being compiled, the exe files runs without mistake, and the video can also be played. I don't know what causes the difference?
By the way, what can i do to load the video file successfully when i push the play button without compiling it into exe file. Sorry for my poor English! Here is the code

 using UnityEngine;
 using System.Collections;
 using System.IO; // To use the Directory class
 using System.Collections.Generic; // To use the List class
 
 [System.Serializable] // To make List objects visible in the inspector
 public class MovieLoader : MonoBehaviour
 { 
     public List<MovieTexture> allVideos;
     public int videoIndex = 0; // Default video to play
 
     public int videoCount = 0;
 
 
     void Start ()
     {
         StartCoroutine(LoadAllVideos());
     }
 
 
     IEnumerator LoadAllVideos()
     {
         allVideos = new List<MovieTexture>();
       //  string[] videoFilePaths = Directory.GetFiles("E:\\Videos", "*.ogv");
         string[] videoFilePaths = Directory.GetFiles("F:\\Resources", "*.ogv");
 
         foreach (string file in videoFilePaths) {
             Debug.Log (file);
             videoCount++;
         }
 
         for (int i = 0; i < videoFilePaths.Length; i++)
         {
 
             WWW diskMovieDir = new WWW("file:///" + videoFilePaths[i]);
 
             //Wait for file finish loading
             while (!diskMovieDir.isDone)
             {
                 yield return null;
             }
             //Save the loaded movie from WWW to movetexture
             allVideos.Add(diskMovieDir.movie);
         }
         MovieToPlane();
     }
 
 
     public void MovieToPlane()
     {
         Renderer rend;
         rend = GetComponent<Renderer>();
         rend.material.SetTexture("_MainTex", allVideos[videoIndex]);
         allVideos[videoIndex].Play();
         allVideos[videoIndex].loop = true;
     }
 
     public void ChangeVideo()
     {
         allVideos[videoIndex].Stop();
         Renderer rend;
         rend = GetComponent<Renderer> ();
         videoIndex = (videoIndex + 1) % videoCount;
         rend.material.SetTexture("_MainTex", allVideos[videoIndex]);
 
         Debug.Log(allVideos [videoIndex].duration.ToString());
         allVideos[videoIndex].Play();
         allVideos[videoIndex].loop = true;
     }
 }