C# code for detaching a sql server database

I am using the following C# code to establish a SQL connect to a SQL
database file:



// connection string

// attach a SQL database file to a local SQL server express instance

string _connectionString = "Data Source=.\\SQLEXPRESS;AttachDbFilename="+ GlobalClass.app.ToString()+"\\dbRDAgent.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True";


// using System.Data.SqlClient;

SqlConnection _sqlConnection = new SqlConnection(_connectionString);

// open the connection

_sqlConnection.Open();



// do something



// close the connection

_sqlConnection.Close();



So far, the connection works fine.



However, next, I want to copy the database file to another folder. So
the following codes:

// source database file name

string sourceDatabaseFileName = GlobalClass.app.ToString()+"\\dbRDAgent.mdf;

// target database file name
string targetDatabaseFileName = "D:\BalanceDatabase_1.mdf";

// copy database file

System.IO.File.Copy(sourceDatabaseFileName, targetDatabaseFileName,
true);



Then the program came with runtime exception: "IOException was
unhandled: The process cannot access the file 'C:
\BalanceDatabase_1.mdf' because it is being used by another process."



Is it because the database file was sill attached to the local SQL
Server express instance? What can I do to bypass this problem? Detach
the database file? or dispose the local SQL Server express instance?