how to fix : Cannot open database "C:\EasyFactorDB.mdf" requested by the login. The login failed. Login failed for user 'REDAPPLE6\redapple'

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SQL

C#5.0

hello to everyone.
please help me.
I have a windows application with c# and sql Express 2008.
In my application,I have database backup and restore function.
my backup function execute without any error,but my restore function work on my computer and don't work on end user system.
error on end user system:

Cannot open database "C:\EasyFactorDB.mdf" requested by the login. The login failed. Login failed for user 'REDAPPLE6\redapple'

my connection string :
Data Source=.\\SQLEXPRESS;Initial Catalog=C:\EasyFactorDB.mdf;Integrated Security=True;User Instance=True

my restore function:

public bool RestoreDatabase(string BackUpFolderName, string BackUpFileName, string DestinitionPath)
{
SqlConnection sc = new SqlConnection("Data Source=.\\SQLEXPRESS;Initial Catalog=" + DestinitionPath + ";Integrated Security=True;User Instance=True");
sc.Open();

Server myServer = new Server(sc.DataSource);
myServer.ConnectionContext.LoginSecure = true;
myServer.ConnectionContext.MultipleActiveResultSets = true;
myServer.ConnectionContext.Connect();
Database myDatabase = myServer.Databases[sc.Database];
Restore restoreDB = new Restore();
restoreDB.Action = RestoreActionType.Database;
restoreDB.Database = myDatabase.Name;

string FilePath = BackUpFolderName + Path.DirectorySeparatorChar + BackUpFileName;

if (!System.IO.File.Exists(FilePath))
{
return false;
}

myServer.KillAllProcesses(sc.Database);

restoreDB.Devices.AddDevice(FilePath, DeviceType.File);

if (!restoreDB.SqlVerify(myServer))
{
return false;
}

restoreDB.SqlRestore(myServer);

if (myServer.ConnectionContext.IsOpen)
{
myServer.ConnectionContext.Disconnect();
}

sc.Close();

return true;
}