Restore database Error: The file cannot be overwritten. It is being used by another database

Dear sir,

I used following c# code I got error about single user

The file 'C:\Program Files\Microsoft SQL Server\MSSQL10_50.MSSQLSERVER\MSSQL\DATA\testvms.mdf' cannot be overwritten. It is being used by database 'vmst'.

File 'VMS' cannot be restored to 'C:\Program Files\Microsoft SQL Server\MSSQL10_50.MSSQLSERVER\MSSQL\DATA\testvms.mdf'. Use WITH MOVE to identify a valid location for the file.

The file 'C:\Program Files\Microsoft SQL Server\MSSQL10_50.MSSQLSERVER\MSSQL\DATA\testvms_log.ldf' cannot be overwritten. It is being used by database 'vmst'.

File 'VMS_log' cannot be restored to 'C:\Program Files\Microsoft SQL Server\MSSQL10_50.MSSQLSERVER\MSSQL\DATA\testvms_log.ldf'. Use WITH MOVE to identify a valid location for the file.

Problems were identified while planning for the RESTORE statement. Previous messages provide details.

RESTORE DATABASE is terminating abnormally.

Changed database context to 'master'.

private void restoreButton_Click(object sender, EventArgs e)
{
    string database = con.Database.ToString();
    if (con.State != ConnectionState.Open)
    {
        con.Open();
    }
    try
    {
        string sqlStmt2 = string.Format("ALTER DATABASE [" + database + "] SET SINGLE_USER WITH ROLLBACK IMMEDIATE");
        SqlCommand bu2 = new SqlCommand(sqlStmt2, con);
        bu2.ExecuteNonQuery();

        string sqlStmt3 = "USE MASTER RESTORE DATABASE [" + database + "] FROM DISK='" + textBox2.Text + "'WITH REPLACE;";
        SqlCommand bu3 = new SqlCommand(sqlStmt3, con);
        bu3.ExecuteNonQuery();

        string sqlStmt4 = string.Format("ALTER DATABASE [" + database + "] SET MULTI_USER");
        SqlCommand bu4 = new SqlCommand(sqlStmt4, con);
        bu4.ExecuteNonQuery();

        MessageBox.Show("database restoration done successefully");
        con.Close();
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.ToString());
    }
}

Kindly help me