我有一个属性,(下面显示示例)。
[DefaultValue(false)]
public bool MyProperty {
get {
return myVal;
}
set {
myVal=value;
}
}
我正在使用的情况是,如果未设置默认值,请确保它在PropertyGrid中显示为bold。
我感到非常烦人的是,在我的构造函数中,我必须设置属性的初始值,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,并进行相应的设置?类似于:
myctor()
{
myVal = GetDefaultValueProperty<bool>("MyProperty");
}
您可以使用以下代码获取所需的元数据。
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;
}
}
如果您想做某事确实很酷,则可以使用Lambda表达式进行此操作:
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;
}
}
当时的用法是:
myctor()
{
myVal = GetDefaultValue(x => x.MyProperty);
}
您可以调用GetProperty
方法来找到属性,然后调用GetCustomAttributes(typeof(DefaultValueAttribute)
(并转换其结果)以应用属性。
这是基于Paul Turner的回答的通用扩展方法。它适用于任何班级的任何成员。
public static bool TryGetDefaultValue<TSource, TResult>(this TSource _, Expression<Func<TSource, TResult>> expression, out TResult result)
{
if (((MemberExpression)expression.Body).Member.GetCustomAttribute(typeof(DefaultValueAttribute)) is DefaultValueAttribute attribute)
{
result = (TResult)attribute.Value;
return true;
}
result = default;
return false;
}