What does this code do: using (SqlConnection cn = new SqlConnection(connectionString))

What does

using (Sq开发者_如何学ClConnection cn = new SqlConnection(connectionString))

do?

new SqlConnection(connectionString)

creates a new SqlConnection instance against the supplied connection string.

SqlConnection cn = ...

assigns it to the new local variable cn (scoped to the using statement) that holds the constructed connection object.

using(...)

Is a using statement - it ensures that the connection is Dispose()-d at the end, even if an exception is thrown (in this case Dispose() means closing it / releasing to the pool etc)

The whole code is essentially:

{ // this { } scope is to limit the "cn"
    SqlConnection cn = new SqlConnection(connectionString);
    try { // the body of the using block
        ...
    } finally { // dispose if not null
        if(cn != null) { cn.Dispose(); }
    }
}

It disposes the SqlConnection after it's not needed anymore. In this case SqlConnection can leave some unmanaged resources behind and you should clean them up. SqlConnection implements IDisposable, which means you can (should) call Dispose after you are done working with it.

It's basically a shorthand for:

try {
    SqlConnection cn = new SqlConnection(connectionString);

    // whatever other code
}
finally {
    if (cn != null)
    {
        cn.Dispose();
    }
}

When you use using, scope of cn is extended also outside try (to finally).

If you want to learn more, check MSDN's article regarding this topic.

Makes sure the SqlConnection is destroyed at the end of the "using" scope

the using statement can be applied to objects that implement the IDisposable interface. At the end of the using scope Dispose is called on the object.

Here are some links to the MSDN doco:

Using statement.

SqlConnection class.

This ensures releasing connection when control leaves using() {..} clause.

Leaving may occur as a result of exception, function return, break from loop, goto or normal leaving of scope. Very convenient.