Get the default value of a property given its name

Question

I have a property, (example shown below).

 [DefaultValue(false)]
 public bool MyProperty {
    get {
       return myVal;
    }
    set {
       myVal=value;
    }
 }

The situation I'm using this is to make sure it shows up as bold in the a PropertyGrid if the default value is not set.

I find it incredibly annoying that in my constructor, I have to set the initial value of my property, and hope that they match up.

Is it possible to have my constructor "discover" the default value of a given property, and set it accordingly? Something like:

myctor()
{
   myVal = GetDefaultValueProperty<bool>("MyProperty");
}

Answer 1

You can use the following code to get the metadata you're after.

public static T GetDefaultValue<T>(string propertyName)
{
    var property = typeof(MyClass).GetProperty(propertyName);

    var attribute = property
        .GetCustomAttribute(typeof(DefaultValueAttribute)) 
            as DefaultValueAttribute;

    if(attribute != null)
    {
        return (T)attribute.Value;
    }
}

---

If you want to do something really cool, you can do this with a Lambda expression:

public static T GetDefaultValue<T>(
    Expression<Func<T, MyClass>> propertySelector)
{
    MemberExpression memberExpression = null;

    switch (expression.Body.NodeType)
    {
        case ExpressionType.MemberAccess:
            // This is the default case where the 
            // expression is simply member access.
            memberExpression 
                = expression.Body as MemberExpression;
            break;

        case ExpressionType.Convert:
            // This case deals with conversions that 
            // may have occured due to typing.
            UnaryExpression unaryExpression 
                = expression.Body as UnaryExpression;

            if (unaryExpression != null)
            {
                memberExpression 
                    = unaryExpression.Operand as MemberExpression;
            }
            break;
    }


    MemberInfo member = memberExpression.Member;

    // Check for field and property types. 
    // All other types are not supported by attribute model.
    switch (member.MemberType)
    {
        case MemberTypes.Property:
            break;
        default:
            throw new Exception("Member is not property");
    }

    var property = (PropertyInfo)member;

    var attribute = property
        .GetCustomAttribute(typeof(DefaultValueAttribute)) 
            as DefaultValueAttribute;

    if(attribute != null)
    {
        return (T)attribute.Value;
    }
}

Usage is then:

myctor()
{
   myVal = GetDefaultValue(x => x.MyProperty);
}