根据名称获取属性的默认值

Question

我有一个属性,(如下所示).

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

我正在使用它的情况是确保它在PropertyGrid中显示为粗体,如果未设置默认值.

我觉得令人难以置信的是,在我的构造函数中,我必须设置我的属性的初始值,并希望它们匹配.

是否可以让我的构造函数"发现"给定属性的默认值,并相应地设置它？就像是:

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

Answer 1

您可以使用以下代码获取您所追求的元数据.

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;
    }
}

如果你想做一些非常酷的事情,可以使用Lambda表达式执行此操作:

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;
    }
}

用法是:

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

Answer 2

您可以调用该GetProperty方法来查找属性，然后调用GetCustomAttributes(typeof(DefaultValueAttribute)（并转换其结果）以获取应用的属性。