Detach Database File From Local SQL Server Express Instance By C#?

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 = @"Server=.SQLExpress; AttachDbFilename=C:BalanceDatabase_1.mdf; Trusted_Connection=Yes; 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 = @"C:BalanceDatabase_1.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?

Many thanks indeed!