任何人都知道一个快速的方法去自定义属性的枚举值？ [英] Anyone know a quick way to get to custom attributes on an enum value?

问题描述

这可能是最好的一个例子所示。我有属性的枚举：

This is probably best shown with an example. I have an enum with attributes:

public enum MyEnum {

    [CustomInfo("This is a custom attrib")]
    None = 0,

    [CustomInfo("This is another attrib")]
    ValueA,

    [CustomInfo("This has an extra flag", AllowSomething = true)]
    ValueB,
}

我想从一个实例的属性：

I want to get to those attributes from an instance:

public CustomInfoAttribute GetInfo( MyEnum enumInput ) {

    Type typeOfEnum = enumInput.GetType(); //this will be typeof( MyEnum )

    //here is the problem, GetField takes a string
    // the .ToString() on enums is very slow
    FieldInfo fi = typeOfEnum.GetField( enumInput.ToString() );

    //get the attribute from the field
    return fi.GetCustomAttributes( typeof( CustomInfoAttribute  ), false ).
        FirstOrDefault()        //Linq method to get first or null
        as CustomInfoAttribute; //use as operator to convert
}

由于这是使用反射我期待一些缓慢，但它似乎凌乱转换枚举值的字符串（它反映了名称）时，我已经有它的一个实例。

As this is using reflection I expect some slowness, but it seems messy to convert the enum value to a string (which reflects the name) when I already have an instance of it.

有没有人有更好的办法吗？

Does anyone have a better way?

推荐答案

这可能是最简单的方法。

This is probably the easiest way.

有一个更快的方法是静态放出使用动态方法和的ILGenerator的IL code。虽然我只用它来为getPropertyInfo，但不明白为什么你不能发出CustomAttributeInfo为好。

A quicker way would be to Statically Emit the IL code using Dynamic Method and ILGenerator. Although I've only used this to GetPropertyInfo, but can't see why you couldn't emit CustomAttributeInfo as well.

例如code从物业发出的getter

For example code to emit a getter from a property

public delegate object FastPropertyGetHandler(object target);    

private static void EmitBoxIfNeeded(ILGenerator ilGenerator, System.Type type)
{
    if (type.IsValueType)
    {
        ilGenerator.Emit(OpCodes.Box, type);
    }
}

public static FastPropertyGetHandler GetPropertyGetter(PropertyInfo propInfo)
{
    // generates a dynamic method to generate a FastPropertyGetHandler delegate
    DynamicMethod dynamicMethod =
        new DynamicMethod(
            string.Empty, 
            typeof (object), 
            new Type[] { typeof (object) },
            propInfo.DeclaringType.Module);

    ILGenerator ilGenerator = dynamicMethod.GetILGenerator();
    // loads the object into the stack
    ilGenerator.Emit(OpCodes.Ldarg_0);
    // calls the getter
    ilGenerator.EmitCall(OpCodes.Callvirt, propInfo.GetGetMethod(), null);
    // creates code for handling the return value
    EmitBoxIfNeeded(ilGenerator, propInfo.PropertyType);
    // returns the value to the caller
    ilGenerator.Emit(OpCodes.Ret);
    // converts the DynamicMethod to a FastPropertyGetHandler delegate
    // to get the property
    FastPropertyGetHandler getter =
        (FastPropertyGetHandler) 
        dynamicMethod.CreateDelegate(typeof(FastPropertyGetHandler));


    return getter;
}

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