System

System.Configuration

.ApplicationSettingsGroup 
.ClientSettingsSection 
.ConfigurationUserLevel 

System.GC

?? 

System.Guid

System.Guid.NewGuid 

[AttributesUsage] attribute

System.AttributesUsageAttribute 

[Browsable(false)] 
public class MyClass 

Specifies how the custom attribute must be used

[Obsolete] attribute

System.ObsoleteAttribute 

[Obsolete] 
public class MyClass 

[ThreadStatic] attribute

System.ThreadStaticAttribute 

[ThreadStatic] 
public class MyClass