To write dynamic connection string(c#2008,SQL SERVER 2005)

Question

I have used Attach a database file, located in the data directory, on connect to a local SQL Server Express instance.code is
conn = new SqlConnection();
conn.ConnectionString = "Data Source=.\\SQLEXPRESS;AttachDbFilename=|DataDirectory|\\carRent.mdf;Integrated Security=True;User Instance=True";
return conn;
But I get error :

Unable to open the physical file "D:\CarRent Completed\CarRent\bin\Debug\carRent.mdf". Operating system error 32: "32(The process cannot access the file because it is being used by another process.)".
An attempt to attach an auto-named database for file D:\CarRent Completed\CarRent\bin\Debug\carRent.mdf failed. A database with the same name exists, or specified file cannot be opened, or it is located on UNC share.
how to solve this error?Thanks for your time and consideration.