Month: January 2010

Nullable TypeConverter for Silverlight

In Silverlight if you want a property of one of your Controls or Models to be Nullable<T> you will end up with a parser error without a little extra work. The reason for this is that there is no default TypeConverter for Nullable<T> so the parser doesn’t know how to convert the string in the Xaml to the appropriate type. To fix this you simply create your own TypeConverter and apply it to your property.

public class NullableTypeConverter<T> : TypeConverter
    where T : struct
{
    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
    {
        return sourceType == typeof(string);
    }

    public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
    {
        Nullable<T> nullableValue = null;
        string stringValue = value as string;
        if (!string.IsNullOrEmpty(stringValue))
        {
            T converted = (T)Convert.ChangeType(value, typeof(T), culture);
            nullableValue = new Nullable<T>(converted);
        }

        return nullableValue;
    }
}

Then to apply it you simply do the following:

[TypeConverter(typeof(NullableTypeConverter<int>))]
public int? Example { get; set; }

Surprisingly Difficult to Parse a Character

I’ve been working on a custom variant of an OMeta parser for a couple weeks now. It’s coming along pretty well, I think I’ve overcome most of the major hurdles and I’m just trying to go through what I currently have, clean it up and get it to solve some of the edge cases that I need.

Just now I was working on the grammar for parsing a character and realized how hard it really is. It sounds trivial, after all it’s just two single quotes and a character right? Wrong. Here’s my current grammar:

CharacterLiteralToken
    = '\'' '\\' 'u' Hex#4 '\''
    | '\'' '\\' 'U' Hex#8 '\''
    | '\'' '\\' 'x' Hex Hex? Hex? Hex? '\''
    | '\'' '\\' ('\'' | '\"' | '\\' | '0' | 'a' | 'b' | 'f' | 'n' | 'r' | 't' | 'v') '\''
    | '\'' '\u0000'..'\uffff' '\'';

It turns out that you have to be sure to account for a multitude of escape characters as well as escaped Unicode literals. I didn’t want to have to implement this, but you can see the last rule which just matches every character under the sun needed it.

This will match:

  • ‘\u0000’
  • ‘\U00000000’
  • ‘\x0’, ‘\x00’, ‘\x000’, ‘\x0000’
  • ‘\”, ‘\”‘, ‘\\’, ”, ‘\a’, ‘\b’, ‘\f’, ‘\n’, ‘\r’, ‘\t’, ‘\v’
  • ‘a’ …

Next I get to do the string parser… that should be even more interesting.