C# Attributes and attribute location/targets

I'm reading up about attributes and understand that they can be made to apply to different target entities with you code - (see Attribute Targets).

So, looking at the AssemblyInfo.cs file in my project, I can see the following:

[assembly: AssemblyTitle("AttributesDemo")]
[assembly: AssemblyDescription("")]

Which makes sense to me. An attribute whose target is the assembly.

In my code I can add an attribute on a class as follows:

[MyAttribute]
class MySerialzableClass
{

With MyAttribute being:

[AttributeUsage (AttributeTargets.All)]
public class MyAttribute : System.Attribute
{
}

So, I got to thinking about the assembly: statement in the first code block. And tri开发者_Python百科ed this, just for experimentation:

[class: MyAttribute]
class MySerialzableClass
{

This gives the compiler warning:

'class' is not a recognized attribute location. All attributes in this block will be ignored.

So my question is this - Why do have to specify the Attribute Target on some attributes and are not requried, or permitted to for others? Moreover, for which ones must you do this?

you have to specify target explicitly in case where the target is not represented in code. I know only three that targets, assembly, module and return:

[return: MyAttribute]
public static int meth(

for class specifying class: is excessive, compiler can understand what do you meant

You can specify attribute targets for any attribute usage, but only the ones that don't have a default (assembly and module) are mandatory. Also, you must use these annotations when you want to apply an attribute to a non-default target.

Examples of non-default targets:

[return: MyAttribute]   
public int Method() { ... }

public int Property {
  get;
  [param: MyAttribute] // applies to the parameter to the setter
  set;
}

In your example, the right target (which is the default) is type:

[type: MyAttribute] 
class MySerialzableClass { }

Normally, the attribute comes right before what it affects, such as the class or method. For assembly-wide attributes, there's no "before", so you have to specify.