Add attribute article (#30734)

guide/english/csharp/attributes/index.md

@@ -0,0 +1,68 @@
+---
+title: Attributes
+---
+
+# Attributes
+Attributes allow the programmer to add metadata to assemblies, types, and members. When applied to a type or member, an attribute will take the form of `[Attribute]` or `[Attribute(args)]`. When applied to an assembly, an attribute will take the form of `[assembly:Attribute]` or `[assembly:Attribute(args)]`.
+
+An attribute is any class that inherits from the parent `Attribute` type. Conventionally, the name of an attribute in its definition will be `SomethingAttribute`, rather than just `Something`. When used, `[Something]` will still compile:
+```csharp
+public class PluginAttribute : Attribute
+{
+	// Attribute members
+}
+
+[Plugin]
+public class MyPlugin
+{
+	// Plugin members
+}
+```
+
+As mentioned above, attributes can have constructors like any other class. However, all arguments in an attribute constructor must be compile-time constants.
+```csharp
+public static class Variables // For the purposes of demonstration
+{
+	public static string MyPluginName = "Cool Plugin";
+	public const string MyConstPluginName = "Amazing Plugin";
+}
+
+public class PluginAttribute : Attribute
+{
+	public string Name { get; private set; }
+
+	public PluginAttribute(string name)
+	{
+		Name = name;
+	}
+}
+
+[Plugin(MyPluginName)] // Won't compile because MyPluginName isn't const
+[Plugin(MyConstPluginName)] // OK
+[Plugin("My Cool Plugin")] // OK
+public class MyPlugin
+{
+	// Plugin members
+}
+```
+
+## Accessing a Type's Attributes
+The `System.Attribute.GetCustomAttributes(Type)` method returns an array of all the attributes applied to a type. The programmer can then loop through this array to find the desired attribute using the `is` keyword.
+```csharp
+public void PrintPluginName()
+{
+	var type = typeof(MyPlugin); // Returns a Type object representing our MyPlugin class
+	var attributes = System.Attribute.GetCustomAttributes(type); // Returns an Attribute[]
+
+	foreach (var a in attributes)
+	{
+		if (a is PluginAttribute plugin)
+			Console.WriteLine($"Plugin Name: {plugin.Name}");
+	}
+}
+```
+
+#### Additional Information
+* [Attributes - Microsoft Programming Guide](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/attributes/)
+* [Accessing Attributes Using Reflection - Microsoft Programming Guide](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/attributes/accessing-attributes-by-using-reflection)
+* [Creating Custom Attributes - Microsoft Programming Guide](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/attributes/creating-custom-attributes)