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    <p>The issue is with the constructor arguments. They need to come from <em>somewhere</em>, they are not supplied by code that consumes the attribute. They must be supplied by the Reflection plumbing when it creates the attribute object by calling its constructor. For which it needs the constructor argument values.</p> <p>This starts at compile time with the compiler parsing the attribute and recording the constructor arguments. It stores those argument values in the assembly metadata in a binary format. At issue then is that the runtime needs a highly standardized way to deserialize those values, one that preferably doesn't depend on any of the .NET classes that you'd normally use the de/serialize data. Because there's no guarantee that such classes are actually available at runtime, they won't be in a very trimmed version of .NET like the Micro Framework. </p> <p>Even something as common as binary serialization with the BinaryFormatter class is troublesome, note how it requires the [Serializable] attribute on the class to allow it to do its job. Versioning would also be an enormous problem, clearly such a serializer class could <em>never</em> change for the risk of breaking attributes in old assemblies. </p> <p>This is a rock and a hard place, solved by the CLS designers by heavily restricting the allowed types for an attribute constructor. They didn't leave much, just the simple values types, string, a simple one-dimensional array of them and Type. Never a problem deserializing them since their binary representation is simple and unambiguous. Quite a restriction but attributes can still be pretty expressive. The ultimate fallback is to use a string and decode that string in the constructor at runtime. Creating an object of MyClass isn't an issue, you can do so in the attribute constructor. You'll have to encode the arguments that this constructor needs however as properties of the attribute.</p>
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    1. POIf attributes are only constructed when they are reflected into, why are attribute constructors so limited?
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    1. COCouldn't you serialize type names as strings and then do something like this: [MyAttribute(new MyType(3, "string constant", new MyOtherType(4, null, "test")))] ?
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    2. CONot so sure what you are proposing. But you'll have to stop using *new* in attribute declarations, code is not supported either. Only literals. As stated, if you need to create an object then you need to do so in the attribute class constructor. With fairly high odds that you are on the wrong track because the code that reads the attribute back won't know beans about the class you create. Do focus a bit on how you are going to *use* the attribute, that's most often overlooked.
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    3. COWhat I'm saying is why isn't that sort of thing possible?
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