Exception coming as System.MethodAccessException

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XNA4.0

ITS WINDOWS PHONE APP...

FileStream fileread;
FileStream filewrite;
StreamReader scoreread;
StreamWriter scorewitte;
public void gethighscxore()
{
Boolean boolwork = true;

// create fo
fileread = new FileStream("ok.txt", FileMode.Open, FileAccess.Read);

scoreread = new StreamReader(fileread);

for (int i = 0; i < maxhscore; i++)
{
texthscore1[i] = scoreread.ReadLine();

if (texthscore1[i] == null)
{
texthscore1[i] = "0";
}
}

scoreread.Close();
fileread.Close();

if (boolwork)
{
int j = 0;

for (int i = 0; i < maxhscore; i++)
{
if (plane_.score > Convert.ToInt32(texthscore1[i]) && i == j)
{
texthscore2[i] = plane_.score.ToString();
i++;
if (i < maxhscore)
{
texthscore2[i] = texthscore1[j];
}
}
else
{
texthscore2[i] = texthscore1[j];
}
j++;
}
}
filewrite = new FileStream("ok.txt", FileMode.Create, FileAccess.Write);

scorewitte = new StreamWriter(filewrite);

for (int i = 0; i < maxhscore; i++)
{
scorewitte.WriteLine(texthscore2[i]);

}
scorewitte.Close();
filewrite.Close();
}

this is the proper code..this couch exception error,,,
System.MethodAccessException
i think ther is any problem in fileaccesing...
pls help me how can i resolve it