What is the benefit of "using"

Question

I want to understand the difference between

public DataTable ExectNonActQuery(string spname, SqlCommand command)
{
    using (DataTable dt = new DataTable())
    {
        cmd = command;
        cmd.Connection = GetConnection();
        cmd.CommandText = spname;
        cmd.CommandType = CommandType.StoredProcedure;
        da.SelectCommand = cmd;
        da.Fill(dt);
        return (dt);
    }
}

and

public DataTable ExectNonActQuery(string spname, SqlCommand command)
{
    DataTable dt = new DataTable();
    cmd = command;
    cmd.Connection = GetConnection();
    cmd.CommandText = spname;
    cmd.CommandType = CommandType.StoredProcedure;
    da.SelectCommand = cmd;
    da.Fill(dt);
    return (dt);
    }
}

I actually want to understand what is the benefit of creating a new object using "using" instead of creating it directly like this

DataTable dt = new DataTable();

Answer 1

Your first example is incorrect: using(var ...) ensures that IDisposable.Dispose is called on the variable declared inside using upon exiting. Therefore your first example returns a disposed object, which is almost certainly wrong.

In general, the using block is roughly equivalent to this:

var x = new MyDisposable();
try {
    // Do something with x
} finally {
    x.Dispose();
}

Since this is a very common pattern in C#, a special syntax is provided for your convenience.