Why does ConfigurationValidator validate the default value of a ConfigurationProperty even if IsRequired is true?

I have had this problem before. There was a valid reason for this but I cannot remember the details.

I cannot remember if this works but you can try declaring the property in the constructor where null is the default value.

public class CustomConfigurationSection : ConfigurationSection
{
    public CustomConfigurationSection()
    {
        Properties.Add(new ConfigurationProperty(
            "x",
            typeof(string),
            null,
            null,
            new StringValidator(1),
            ConfigurationPropertyOptions.IsRequired));
    }


    public string X
    {
        get { return (string)this["x"]; }
        set { this["x"] = value; }
    }
}

This is related to using default values and validators but is where a default value is wanted. http://msdn.microsoft.com/en-us/library/system.configuration.configurationproperty(VS.85).aspx#1

EDIT

I have just tried out the previous code and it does as I expected. My previous code did not compile as I missed out a constructor property so I have fixed that.

The reason is that the configuration section classes can be newed up in code without there being a configuration file. You can use a default constructor and not specify any values for the property. In this case, even if you have specified IsRequired=true there is no exception thrown. In other words, IsRequired only applies if the property is being deserialized from XML.

However, DefaultValue does apply in this case, as it does when the property is being deserialized from XML (as does any ConfigurationValidatorAttribute).

This makes sense if you are using configuration sections in unit testing. It's really nice to A) have a declarative default value when constructing the section and B) have the default value validated.