Attributes, just metadata or needed?

Few days ago I asked what this attribute means:

 [System.Runtime.InteropServices.DllImport("KERNEL32.DLL", EntryPoint="RtlZeroMemory")]  public unsafe static extern bool ZeroMemory(byte* destination, int length); 

I have learned that attributes are metadata but what I do not understand is 开发者_JAVA技巧- is this needed in this case? I thought metada are just that, metadata that can be ommited. Also the code seems to be running fine when I remove the attibute. I would like to understand.

PS: Hans Passant mentioned its covered by any book about .NET Csharp..it is not, the largely used one VS 2010 from John Sharp does not cover it.

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The metadata does usually have a reason and a meaning. In this particular case it tells the compiler how to bind this external method definition (e.g. to which DLL import it matches).

Other attributes control how interop is performed by the framework, yet other control how the object inspector displays data. 3rd-party attributes are also used extensively to control various behaviors, for instance for finding specific type information when performing reflection.

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No, this attribute is absolutely required. It informs the CLR that what you've defined actually uses platform invokation services (or, P/Invoke) to call a function defined in unmanaged code.

Specifically, the RtlZeroMemory function, defined in the library kernel32.dll.

Without it, the compiler wouldn't know which function it was bound to, and the CLR wouldn't know which function to call at run-time.

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This attribute is doing 2 things

• Informs the CLR that the C method being invoked lives in kernel32.dll
• Informs the CLR that the C method name is RtlZeroMemory and not ZeroMemory as it's named in code.

Yes this attribute is 100% necessary. It's a requirement for any PInvoke method to at the least name the DLL the C method lives in.

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As your example shows, attributes are in fact needed in several key areas of .NET programming.

Attributes provide a model known as "Aspect-Oriented Programming" or AOP. Instead of having to write code that performs some specific task, such as serialization, DLL interop, logging, etc, you can instead simply decorate the classes or members on which you want these tasks performed with an attribute. Attributes are a special type of class, with members which can be invoked by the CLR as it runs your code, that will perform the task you wanted when you decorated the code.

You are correct in part; many attributes are intended simply to store metadata. DescriptionAttribute is a good one. However, even in this case, the attribute is important depending on how it's used. If you are decorating a member of a GUI class that you want to use in the designer, [Description()] provides valuable information to the user of the class in the designer, which may not be you. I've also seen and used many alternate uses for DescriptionAttribute; it can be applied to almost anything, so I've used it to provide "friendly names" for Enum constants, coupled with a GetDescription() extension method to grab them, when using Enums to populate drop-down lists.

So, while it's technically "metadata", an attribute's being "required" is governed by how much you want the task inherent in that attribute to be performed.

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As for this particular attribute, I'm not too sure. To be honest, I've never seen it in almost a year of constant C#.

However, attributes in general can prove very useful. For instance, I was having issues with the VS2010 designer setting autocomplete properties in the wrong order, and getting run-time errors as a result. The solution was to add attributes to the autocomplete properties that prevented the designer from writing these properties to the design file, and instead setting the properties myself in the .cs file (in the proper order).

Summary: Attributes (usually) are not required, but can prove extremely useful.