Meaning of text between square brackets

Question

I have seen a lot of C# programs that use the [], for example [STAThread] and then the code follows. Another classic example is [DLLImport].

I know what STAThread means but my question is what is the significance of the square brackets, essentially what do they tell the compiler?

Answer 1

It's an attribute. Attributes are a form of metadata that you can attach to various code elements: classes, methods, assemblies etc.

Some attributes have special meaning to the C# compiler, for instance the [Serializable] probably tells the compiler to emit some code that can serialize an instance of the class (I say 'probably' since I do not know the inner workings of the C# compiler).

You can also create your own attributes (by inheriting System.Attribute). Using reflection you could then at run-time extract information from the attributes.

A simple example would be to create an attribute to specify what kind of input field to use in a HTML form when displaying an object's property.

Some links:

• Book chapter on attributes
• Attributes overview (MSDN)
• https://stackoverflow.com/search?q=C%23+attributes
• http://www.google.com/search?q=C%23+attributes

Answer 2

These are attributes.

Attributes have many uses - [Obsolete] marks a method as obsolete and the compiler will warn you. Others like [DebuggerNonUserCode] tell nothing to the compiler and are there to let the debugger know that the code in the marked method is auto-generated.

You can also create your own attributes and use them to mark any kind of metadata. For example, your Customer object might have an attribute [MarketingInformation("Customer is rich! Milk him good!")].