我在做忘记密码，我写下面的代码.但是我得到了错误.那就是size属性的大小为0.

public string forgotpassword(string tutor_username, string tutor_email)
{
    SqlConnection con = new SqlConnection(StrCon);
    SqlCommand cmd = new SqlCommand();
    con.ConnectionString = StrCon;
    //DataSet resultds = new DataSet();
    cmd.Connection = con;
    cmd.CommandType = CommandType.StoredProcedure;
    //SqlDataAdapter da = new SqlDataAdapter();
    cmd.CommandText = "tutor_Retrievepassword";
    cmd.Parameters.Add("@tutor_username", SqlDbType.NVarChar).Direction = ParameterDirection.Input;
    cmd.Parameters.Add("@tutor_password", SqlDbType.NVarChar).Direction = ParameterDirection.Output;
    cmd.Parameters.Add("@tutor_email", SqlDbType.NVarChar).Direction = ParameterDirection.Input;
    cmd.Parameters["@tutor_username"].Value = tutor_username;
    //cmd.Parameters["@password"].Value = password;
    cmd.Parameters["@tutor_email"].Value = tutor_email;
    // da.SelectCommand = cmd;
    
    try
    {
        con.Open();
        cmd.ExecuteReader();
        return cmd.Parameters["@tutor_password"].ToString();
        // da.Fill(resultds);
    }
    catch (Exception ex)
    {
        throw ex;
        //  return null;
    }
    finally
    {
        cmd.Dispose();
        con.Close();
        con.Dispose();
    }
    //return resultds;
}