Question

这是我的属性定义：

[AttributeUsage(AttributeTargets.Field)]
public class Optional : System.Attribute
{
    public Optional()
    {
    }
}

在MyClass中：

[Optional] public TextBox Name;

最后在另一个功能中：

typeof(MyClass).GetFields().ToList<FieldInfo>().ForEach(x => writer.WriteLine(
   x.FieldType + " is called " + 
   x.Name + " and has attributes " + 
   x.GetCustomAttributes(true)[0]
 ));

问题是我收到索引0的错误。我只想检查应用属性的字段。当我删除x.GetCustomAttributes(true)[0]时错误消失。

确切错误：

异常详细信息：System.IndexOutOfRangeException：索引超出了数组的范围。

来源错误：

Line 63:             }
Line 64: 
Line 65:             typeof(T).GetFields().ToList<FieldInfo>().ForEach(x => writer.WriteLine(x.FieldType + " is called " + 
Line 66:                 x.Name + " and has attributes " + 
Line 67:                 x.GetCustomAttributes(true)[0]+ "</br>"));

Answer 1

似乎这里可能有两个问题。要查找具有[可选]属性的所有字段，您需要：

typeof(MyClass).GetFields().Where(
   f => f.GetCustomAttributes(typeof(OptionalAttribute), true).Any())

要在所有字段上写出自定义属性，您需要：

typeof(MyClass).ToList<FieldInfo>().ForEach(x => 
{
 writer.WriteLine(
   x.FieldType + " is called " + 
   x.Name + " and has attributes " + 
   string.Join(", ", x.GetCustomAttributes(true).Select(a => a.ToString()).ToArray()));
});

Answer 2

正如您似乎已经收集到的那样，您收到此错误是因为您正在尝试获取空数组的第0个元素，这当然是非法的。您需要首先过滤掉没有任何属性的字段，如下所示：

var fields = from fieldInfo in typeof(MyClass).GetFields()
             let Attributes = fieldInfo.GetCustomAttributes(true)
             where Attributes.Any()
             select new { fieldInfo.FieldType, fieldInfo.Name, Attributes };

foreach (var field in fields)
{
    writer.WriteLine(field.FieldType + " is called " +
                     field.Name + " and its first attribute is " +
                     field.Attributes.First());
}

如果您对那些OptionalAttribute的人特别感兴趣，那么您可能正在寻找类似的东西：

var fields = from fieldInfo in typeof(MyClass).GetFields()
             let attributes = fieldInfo.GetCustomAttributes(true).OfType<OptionalAttribute>()
             where attributes.Any()
             select new { fieldInfo.FieldType, fieldInfo.Name, Attribute = attributes.Single() };

foreach (var field in fields)
{
    writer.WriteLine(field.FieldType + " is called " +
                     field.Name + " and its OptionalAttribute is " +
                     field.Attribute);
}

Answer 3

检查属性是否存在：

x.GetCustomAttributes(true).Any(item => item is OptionalAttribute)

我假设您将Optional属性重命名为OptionalAttribute，因为所有属性都应具有“属性”后缀。

C＃ - 通过反射枚举自定义属性

3 个答案: