.NET XML Serialization - a settings class

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Introduction

When I first teamed up with Christian the screensaver didn't have a very elegant way of persisting its settings. In the constructor for the screensaver form settings were loaded from the registry, and the same code was copied and pasted into the constructor for the Options dialog.

After my failure of trying to speed up the drawing process (curse you Microsoft for making CachedBitmap an internal class), the first task I brought myself to was to try to clean up the settings code. What resulted is the Settings class; which can be used for storing settings for the user or application.

This class differs slightly from the one in the screensaver because it has been refactored a bit but the premise behind it is the same.

Class make-up

Unless specified, members are assumed to be public instance.

• Properties
  • string SettingsDirectory
    • (static) This is where settings will be saved if no filename is given to the Save/Load functions. This directory is the default ApplicationData directory for the user.
• Methods
  • void SaveSettingsToFile(Settings settings)
  • void SaveSettingsToFile(string filename, Settings settings)
    • (static) Saves the settings into the file, if a filename isn't specified it defaults to SettingsDirectory\config.dat
  • Settings LoadSettingsFromFile()
  • Settings LoadSettingsFromFile(string filename)
    • (static) Loads the settings from the file, if a filename isn't specified it defaults to SettingsDirectory\config.dat. If the file doesn't exist or there is an error deserializing the file it will return a new Settings object with the default values.
  • void LoadDefaultValues()
    • Loads the default settings into the properties.
• Fields
  • Hashtable settings
    • (protected) Used to store the actual settings, based on a key; the key should be a string representing the property name. This makes updating the class fast and easy because you don't have to mess with private variables.
  • string companyName - (private, static)
    • Contains the Company Name that will be used to create the SettingsDirectory property.
  • string productName - (private, static)
    • Contains the name of the Product that will be used to create the SettingsDirectory property.

How to use the class

1. Alter the class to include your own properties
   1. Add a new public get/set property to the class.
   2. In the set part, have it set the value in the Hashtable using the property name as a key.
   3. In the get part, have it return the value stored in the Hashtable using the property name as the key (you will probably need to cast the value from the Hashtable)
2. Test to ensure there is no problem with the underlying types; if there are no problems then the rest of the article is just information for you to digest. If there were problems then read on to discover why there were problems and how you can work around them.

The XmlSerializer class

The XmlSerializer class does most of the work needed for saving the data in a class to a file. Unlike the other serializers the XmlSerializer requires the classes it serializes to have a public default constructor (a public constructor that takes no parameters) and it only serializes public properties and fields.

Because of this it isn't well suited for classes with lots of internal data, unless those can be completely rebuilt via public properties/field. Because of this limitation, certain types won't be serialized by it, a couple examples are System.Drawing.Color and System.Drawing.Font. I don't have a list of all the classes that fail so you'll just have to use trial and error to find them, if serialization of the class isn't supported an exception will be thrown when you try to create the XmlSerializer or the resulting serialization will be an empty tag. I describe a couple techniques to work around this, and I provided fixex for both Color and Font classes so they work, with little effort.

Serializing and deserializing a class with the XmlSerializer is really easy, because it is I'm not going to spend much time on it; but instead I'll spend most of my time describing how to work around its limitations. Simply create a new instance of the XmlSerializer class passing in the Type that corresponds to your class.

XmlSerializer xs = new XmlSerializer(typeof(Settings));

To serialize a class, call Serialize() and pass in a Stream and an instance of the class you wish to serialize. To deserialize, call Deserialize() and pass in the Stream to read from, and cast the return value back to your class type.

Working around the limitations

Custom parsing

System.Drawing.Color is the first limitation I worked around. I did this by first deciding how I was going to store the data in the XML file. I chose a string delimited by colons (:). This string is made up of two parts, the first part tells what type of data follows, either a color name or ARGB values. The second part is either a name or the values split up by colons.

First I created an enumeration that would signify each type.

public enum ColorFormat
{
    NamedColor,
    ARGBColor
}

Then with Color in hand I can reference the IsNamedColor property to decide which of the two formats to return. I do that with this bit of code.

public string SerializeColor(Color color)
{
    if( color.IsNamedColor )
        return string.Format("{0}:{1}", 
            ColorFormat.NamedColor, color.Name);
    else
        return string.Format("{0}:{1}:{2}:{3}:{4}", 
            ColorFormat.ARGBColor, 
            color.A, color.R, color.G, color.B);
}

If you inspect the return value you'll see that what is written for the ColorFormat specifier is the name of the enum value. This causes a slight problem, but is quickly remedied by the Enum class having a Parse method which will convert the name back to a value.

Speaking of reading the values back, here is how I did it. First I take the string in and split it up using colon as the separation character. This results in a string array with 2 or 5 elements in it. I then take the first element and run it through Enum's Parse() method to convert it back to a value suitable for the enum. Then I check that value to determine which of the two formats it is.

public Color DeserializeColor(string color)
{
    byte a, r, g, b;

    string [] pieces = color.Split(new char[] {':'});
		
    ColorFormat colorType = (ColorFormat) 
        Enum.Parse(typeof(ColorFormat), pieces[0], true);

    switch(colorType)
    {
        case ColorFormat.NamedColor:
            return Color.FromName(pieces[1]);

        case ColorFormat.ARGBColor:
            a = byte.Parse(pieces[1]);
            r = byte.Parse(pieces[2]);
            g = byte.Parse(pieces[3]);
            b = byte.Parse(pieces[4]);
			
            return Color.FromArgb(a, r, g, b);
    }
    return Color.Empty;
}

With that code it is now possible serialize a Color object as a string; which the XmlSerializer will handle. How does one go about doing that, and make the resulting XML look as though it handled it natively? By using two attributes you can change the name an element will have in the XML document and tell it ignore other elements as well.

The attribute XmlIgnoreAttribute when applied to a property or field tells the XmlSerializer to ignore that field/property when serializing the class. The attribute XmlElementAttribute does various functions, but the one we're interested in is renaming a serialized element to something else. For our purposes we will apply the XmlIgnore attribute to the original property which can't be serialized; then we'll apply the XmlAttribute to the XmlSerializer friendly property to rename it something more suitable.

A small example

[XmlIgnore()] 
public Color ColorType
{
    get
    {
        return (Color) settings["color"];
    }
    set
    {
        settings["color"] = value;
    }
}

[XmlElement("ColorType")]
public string XmlColorType
{
    get
    {
        return Settings.SerializeColor(ColorType);
    }
    set
    {
        ColorType = Settings.DeserializeColor(value);
    }
}

Here you see that I have a public property named ColorType that will return the Color stored in the settings object. There is also a string property that is used by the XmlSerializer to store the underlying value.

ColorType is the property that the user of the class is to use for setting/retrieving properties. XmlColorType is used by the XmlSerializer to get/set the underlying value and should not be used by the programmer.

Wrapper class/struct

Another way to work around the limitation is to create a class that exposes only the properties needed to recreate the object, and use that to serialize.

This is what I did for the Font class; exposing the FontFamily, Size, FontStyle, and the GraphicsUnit associated with a Font instance. In keeping with the pattern I used for Color; I supply a property that uses this class, and I have two protected methods which handle moving back and forth between the two types.

XmlFont struct

public struct XmlFont
{
    public string FontFamily;
    public GraphicsUnit GraphicsUnit;
    public float Size;
    public FontStyle Style;

    public XmlFont(Font f)
    {
        FontFamily = f.FontFamily.Name;
        GraphicsUnit = f.Unit;
        Size = f.Size;
        Style = f.Style;
    }

    public Font ToFont()
    {
        return new Font(FontFamily, Size, Style, 
            GraphicsUnit);
    }
}

This is an extremely simple struct. Its entire purpose is to be a lightweight container for the values that will be persisted to the file I don't bother defining properties, and I made it a struct instead of a class so it *is* lightweight. Depending on your needs you could use a heavier implementation; but for this case this works great.

This works because the XmlSerializer will attempt to serialize every property and field, if the property or field is a simple datatype it puts it inline, for structs and classes it serializes its public properties/fields.

Conclusion

I've often wondered why it is that MS didn't let the XmlSerializer work like the Formatters, serializing both public and private data. After toying around with the Settings class a bit I think I discovered why. There isn't a need! The XmlSerializer was made to persist data in a user-readable way, but still be read easily by a program. Publishing private data would be considered a Bad Thing™ and thus would make life difficult for those trying to keep private data private. Though you could go through and add NonSerializable attributes to all your private data, you would be SOL when it came to remoting.

Considering that the workaround for the deficiencies isn't all that difficult to get working I'm coming to agree with Microsoft's decision.

As always bug reports should be posted below, comments or questions can be posted or e-mailed to me.

If you want the XP theme I used in the screenshot, check out the WaterColor Visual Style from ThemeXP.org.

This article, along with any associated source code and files, is licensed under The BSD License