How to get Custom Attribute values for enums?

It is a bit messy to do what you are trying to do as you have to use reflection:

public GPUShaderAttribute GetGPUShader(EffectType effectType)
{
    MemberInfo memberInfo = typeof(EffectType).GetMember(effectType.ToString())
                                              .FirstOrDefault();

    if (memberInfo != null)
    {
        GPUShaderAttribute attribute = (GPUShaderAttribute) 
                     memberInfo.GetCustomAttributes(typeof(GPUShaderAttribute), false)
                               .FirstOrDefault();
        return attribute;
    }

    return null;
}

This will return an instance of the GPUShaderAttribute that is relevant to the one marked up on the enum value of EffectType. You have to call it on a specific value of the EffectType enum:

GPUShaderAttribute attribute = GetGPUShader(EffectType.MyEffect);

Once you have the instance of the attribute, you can get the specific values out of it that are marked-up on the individual enum values.

There is another method to do this with generics:

public static T GetAttribute<T>(Enum enumValue) where T: Attribute
{
    T attribute;

    MemberInfo memberInfo = enumValue.GetType().GetMember(enumValue.ToString())
                                    .FirstOrDefault();

    if (memberInfo != null)
    {
        attribute = (T) memberInfo.GetCustomAttributes(typeof (T), false).FirstOrDefault();
        return attribute;
    }
    return null;
}

Try using a generic method

Attribute:

class DayAttribute : Attribute
{
    public string Name { get; private set; }

    public DayAttribute(string name)
    {
        this.Name = name;
    }
}

Enum:

enum Days
{
    [Day("Saturday")]
    Sat,
    [Day("Sunday")]
    Sun,
    [Day("Monday")]
    Mon, 
    [Day("Tuesday")]
    Tue,
    [Day("Wednesday")]
    Wed,
    [Day("Thursday")]
    Thu, 
    [Day("Friday")]
    Fri
}

Generic method:

        public static TAttribute GetAttribute<TAttribute>(this Enum value)
        where TAttribute : Attribute
    {
        var enumType = value.GetType();
        var name = Enum.GetName(enumType, value);
        return enumType.GetField(name).GetCustomAttributes(false).OfType<TAttribute>().SingleOrDefault();
    }

Invoke:

        static void Main(string[] args)
    {
        var day = Days.Mon;
        Console.WriteLine(day.GetAttribute<DayAttribute>().Name);
        Console.ReadLine();
    }

Result:

Monday