Question

If connecting to a database consumes a lot of resources, why should a database connection always be closed in your application if you have to open it again? Can I just make this connection available globally throughout my application so that other classes and methods can reuse it?

For example (in pseudo code):

public class PopulateGridViews()
{
    public SqlConnection conn = new SqlConnection(@"Database:DATABASE");
    conn.Open();

    void PopulateGrid1()
    {
        SqlCommand cmd = new SqlCommand("SELECT * FROM TABLE1");
        cmd.Connection = conn;
        cmd.ExecuteNonQuery();
        cmd.Dispose();
        // Populate Grid1
    }

    void PopulateGrid2()
    {
        SqlCommand cmd = new SqlCommand("SELECT * FROM TABLE2");
        cmd.Connection = conn;
        cmd.ExecuteNonQuery();
        cmd.Dispose();
        // Populate Grid2
    }
}

Answer 1

You should not leave connections open.

You should:

1. Open connections as late as possible
2. Close connections as soon as possible

The connection itself is returned to the connection pool. Connections are a limited and relatively expensive resource. Any new connection you establish that has exactly the same connection string will be able to reuse the connection from the pool.

We strongly recommend that you always
close the connection when you are
finished using it so that the
connection will be returned to the
pool. You can do this using either the
Close or Dispose methods of the
Connection object, or by opening all
connections inside a using statement
in C#, or a Using statement in Visual
Basic. Connections that are not
explicitly closed might not be added
or returned to the pool. For more
information, see using Statement (C#
Reference) or How to: Dispose of a
System Resource for Visual Basic. Ref.

You should appropriately wrap anything that implements IDisposable in a using statement block:

 using (SqlConnection connection = new SqlConnection(connectionString))
 {
     connection.Open();

     ...

     command.ExecuteNonQuery();
 }