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Never stop looking for ways to improve

Display Class Objects in CheckedListBox

March 28th, 2010

If you want to use anything more complex than a list of strings in a ListBox, you’re in luck because the control accepts all types of objects.

Custom Objects (Blog Posts) Displayed in a CheckListBox

Custom Objects (Blog Posts) Displayed in a CheckListBox

In this case, I want to display a list of posts found in a blog. Blog is a class which contains Posts, an array of Post classes. To start, I created the CheckedListBox in the form designer, and I add the posts to it like this:

clbPages.Items.AddRange(_blog.Posts);

If I do nothing else, the ListBox will call the Post’s ToString() method and will display as:

Post
Post
Post
Post

We have two options for displaying this correctly:

1. Override the ToString() method. I don’t recommend doing this because ToString() is much more appropriately used in a debugging context.

2. Add a string converter: This will automatically convert each post object to a usable string when called by an object like a ListBox. ListBox uses Convert.ToString() – this uses that converter more appropriately. ToString() should only be used as a fallback.

<pre>
// Use System for the Type object
using System;
// Use ComponentModel for the TypeConvert base class
using System.ComponentModel;

namespace SiteAssistant.Blog
{
    /// <summary>
    /// Converts a post into a list-friendly string, for checkbox lists
    /// </summary>
    class PostConverter : TypeConverter
    {
        /// <summary>
        /// Indicates whether the Post can be converted to a destination type
        /// </summary>
        /// <remarks>
        /// We only support conversions to STRING at present
        /// </remarks>
        /// <param name="context"></param>
        /// <param name="destinationType"></param>
        /// <returns></returns>
        public override bool CanConvertTo(ITypeDescriptorContext context,
            Type destinationType)
        {
            if (destinationType == typeof(string))
                return true;
            else
                return base.CanConvertTo(context, destinationType);
        }

        /// <summary>
        /// Converts the post to the destination type. If the destination
        /// type is not supported, the base Conversion is applied.
        /// </summary>
        /// <remarks>
        /// We only support converting posts to strings at present.
        /// </remarks>
        /// <param name="context"></param>
        /// <param name="culture"></param>
        /// <param name="value"></param>
        /// <param name="destinationType"></param>
        /// <returns></returns>
        public override object ConvertTo(ITypeDescriptorContext context,
            System.Globalization.CultureInfo culture, object value,
            Type destinationType)
        {
            if (destinationType == typeof(string))
            {
                string text = "";
                Post p = value as Post;
                // Ensure that the Post is not null, avoid errors
                if (null != p)
                {
                    text = p.Title;
                }
                return text;
            }
            else
            {
                return base.ConvertTo(context, culture, value, destinationType);
            }
        }
    }
}
</pre>

The code we write in .NET is like the meat inside a sandwich. The framework is the bread that wraps around our logic and keeps our application together. Our new Posts string converter will be called by the application without us needing to override the Convert function.

It doesn’t happen by magic of course. The final change we have to make is to add information about our conversion function to the posts class:

<pre>
    [System.ComponentModel.TypeConverter(typeof(PostConverter))]
    public class Post
    {
        // Rest of the code goes here
    }
</pre>

That’s all there is to it! Now we can pass a list of Posts to the CheckedListBox and manipulate each item directly. In this application, I will be using this technique to provide the Post object to the text editor with a double-click.

Tags: , , ,
Posted in C#, SiteAssistant | No Comments »

SQL Connections in ASP.NET – What you learned is WRONG!

February 15th, 2008

When we learn how to open and use a database connection with ASP.NET, as with any other programming concept in any other programming language, the simplified version used to explain what’s going on is not truly representative of the quality professional code we will one day be expected to write.
Opening and Closing Connections

Case in point: managing of sql database connection resources. How many of us learned to write something like this:


// Create a new SQL Connection object
SqlConnection conn = new SqlConnection( connectionString );

// Open the connection to the database
conn.Open();

// Create a new SQL Command
SqlCommand cmd = new SqlCommand( “DELETE FROM BabyNames;”, conn );

// Execute the command
cmd.ExecuteNonQuery();

// Close the database connection
conn.Close();

Sure it’s easy to follow, but if you deploy that on a moderately busy server you are going to make your client very unhappy.

Dispose Resources

SQLConnection and SQLCommand objects reference unmanaged resources, meaning the C# garbage collector has no framework knowledge about your object. Since these classes both implement the disposable interface it is important to call the Dispose() method in order to correctly free your application’s used memory.

So our code gets updated to look like this:


// Create a new SQL Connection object
SqlConnection conn = new SqlConnection( connectionString );

// Open the connection to the database
conn.Open();

// Create a new SQL Command
SqlCommand cmd = new SqlCommand( “DELETE FROM BabyNames;”, conn );

// Execute the command
cmd.ExecuteNonQuery();

// Dispose of the command
cmd.Dispose();

// Close the database connection
conn.Close();

// Dispose of the connection object
conn.Dispose();

Trap for Errors

What happens if there’s a problem, and your code fails to complete? If your application crashes before your objects are disposed, you are left with the same effect as if you had never disposed your objects at all!

Fortunately, C# includes the try … finally reserved words. If anything within the try { } block fails, the finally { } still executes. We can easily apply this to our program code:


// Create a new SQL Connection object
SqlConnection conn = new SqlConnection( connectionString );

try
{
// Open the connection to the database
conn.Open();

// Create a new SQL Command
SqlCommand cmd = new SqlCommand( “DELETE FROM BabyNames;”, conn );

try
{
// Execute the command
cmd.ExecuteNonQuery();
}
finally
{
// Dispose of the command
cmd.Dispose();
}

// Close the database connection
conn.Close();
}
finally
{
// Dispose of the connection object
conn.Dispose();
}

For my own part, I prefer the using keyword. We can include a using call anywhere we would ordinarily use a disposal object. When the code is compiled, it behaves the same as try … catch, but leaves our program code much more readable.

Even better, we don’t even have to bother calling Dispose() because it does it for us!


// Create a new SQL Connection object
using ( SqlConnection conn = new SqlConnection( connectionString ) )
{
// Open the connection to the database
conn.Open();

// Create a new SQL Command
using ( SqlCommand cmd = new SqlCommand( “DELETE FROM BabyNames;”, conn ) )
{
// Execute the command
cmd.ExecuteNonQuery();
}

// Close the database connection
conn.Close();
}

Slick.

Open Late, Close Early (like a bank)

There is one more thing I would add to this. Creating objects in memory takes time. Although it happens in fractions of a second too fast to be detectable by us, it’s important not to waste processing time wherever possible.

Whenever we Open() a database connection, we expect to use the database right away. If we then create an SqlCommand, we’re wasting the open connection’s time. It’s as if we pick up the phone and listen to the dial tone while we then flip through the white pages looking for the number we want to call.

Let’s change our example code so we will now Open() at the last possible opportunity, and Close() right away when we’ve made our call:


// Create a new SQL Connection object
using ( SqlConnection conn = new SqlConnection( connectionString ) )
{
// Create a new SQL Command
using ( SqlCommand cmd = new SqlCommand( “DELETE FROM BabyNames;”, conn ) )
{
// Open the connection to the database
conn.Open();

// Execute the command
cmd.ExecuteNonQuery();

// Close the connection to the database
conn.Close();
}
}

In the end, the program code we wrote is very similar to the newbie code we started with. However, we’re now protecting our system from memory leaks, and we’re protecting our database from wasted clock cycles. Our code is easy to read and stable.

Tags: , ,
Posted in ASP.NET, Blogging, SQL | 1 Comment »

Breaking Java’s Rules: Instantiating an Interface

February 8th, 2008

Here’s a geeky party trick:

Abstract classes in Java cannot be instantiated. Here we’re going to consider ways in which a button can be programmed within a JPanel. ActionListener is used to react to the button event, but ActionLister is an interface (pure abstract class) so in order to use it you have to derive a class from it and implement actionPerformed.

For example, you can’t do this:

ActionListener buttonListener = new ActionListener();

You have to do this:

public class ButtonListener
{
public void actionPerformed( ActionEvent event )
{
}
}

ButtonListener buttonListener = new ButtonListener();
ourJButton.addActionListener( buttonListener );

However, this code is valid:

ourJButon.addActionListener( new ActionListener() {
public void actionPerformed( ActionEvent event )
{
}
} );

What’s going on? It would appear that we are instantiating a new ActionListener and giving it an actionPerformed() method. We’ve succeeded in giving our button a listener without first creating a class to handle the event.

Anonymous Inner Classes

Of course we aren’t really instantiating ActionListener – it was a trick. What this does is create an anonymous inner class only in this part of our code. Check it out – when you compile your code, javac will create extra class files for our ActionListener.

For one-off buttons, using anonymous inner classes is an excellent way of reducing code bloat and improving the readability of your programs. There are drawbacks of course, which I’ll go into detail in another article, another day.

Tags: , , ,
Posted in Java | No Comments »

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