问题描述

这可能最好用一个例子来说明.我有一个带有属性的枚举:

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,
}

我想从一个实例中获取这些属性:

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，我预计会有些慢，但是当我已经有枚举值的实例时，将枚举值转换为字符串(反映名称)似乎很麻烦.

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.

谁有更好的办法?

推荐答案

这可能是最简单的方法了.

This is probably the easiest way.

一种更快的方法是使用动态方法和 ILGenerator 静态发出 IL 代码.虽然我只将它用于 GetPropertyInfo，但看不出为什么你也不能发出 CustomAttributeInfo.

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.

例如从属性发出 getter 的代码

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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