1. .NET Framework Introduction: Metadata
Ketan Bibave

2. Overview What is Metadata? How is Metadata?
Structure of Metadata Tables
Functions or benefits of Metadata

3. What is Metadata?
The .NET Framework makes component interoperation even easier by allowing compilers to emit additional declarative information into all modules and assemblies. This information, called metadata, helps components to interact seamlessly.
It also known as “self-contained” type library.
Metadata is binary information describing your program that is stored in Managed module.
Every type and member that is defined and referenced in a module or assembly is described within metadata. 

4. How is Metadata? Metadata stores the following information:
Description of the assembly.
Identity (name, version, culture, public key).
The types that are exported.
Other assemblies that this assembly depends on.
Security permissions needed to run.
Description of types.
Name, visibility, base class, and interfaces implemented.
Members (methods, fields, properties, events, nested types).
Attributes.
Additional descriptive elements that modify types and members.
All Metadata in the form of tables and all table internally in the forms of hexadecimal numbers.
Metadata is of two type
DefType (Defined)
RefType (Referenced)
There are only 15 tables divided into above two types of tables.

5. How is Metadata? Metadata Token
A metadata token is a four-byte number.
The top byte denotes the metadata table to which a particular token refers (method, type, and so on). The remaining three bytes specify the row in the metadata table that corresponds to the programming element being described.
If we define a method in C# and compile it into a PE file, the following metadata token might exist in the MSIL portion of the PE file 0x
The top byte (0x06) indicates that this is a MethodDef token. The lower three bytes (000004) tells the common language runtime to look in the fourth row of the MethodDef table for the information that describes this method definition
Metadata also stores information in four heap structures:
string, blob, user string, and GUID.
All the strings used to name types and members are stored in the string heap.
For example, a method table does not directly store the name of a particular method, but points to the method's name stored in the string heap.

6. Structure of Metadata Tables
15 Type of Table
11 Defined Type
7 Module Type
4 Assembly Type
4 Reference Type

7. Structure of Metadata Tables: 11 Defined Type Table
Module Type
ModuleDef 0x00
TypeDef 0x02
FieldDef 0x04
MethodDef 0x06
ParamDef 0x08
PropertyDef 0x17
EventDef 0x14
Assembly Type
AssemblyDef 0x20
FileDef 0x26
ExportedTypeDef 0x27
ManifestResourceDef 0x28

8. Structure of Metadata Tables: 4 Reference Type Table
AssemblyRef 0x23
ModuleRef 0x1
TypeRef 0x01
MemberRef 0x0a

9. Functions or benefits of Metadata
Metadata allows CLR to do Type Safe Verification and hence metadata makes AppDomain possible.
CLR uses metadata for following operations
For object lifetime maintenance and therefore metadata is responsible to automatic garbage collection.
For serialization, deserialization, marshalling and Remote method Invocation using Network data transfer
CLR does Introspection and Reflection.
IDE uses metadata for intellisense feature.
Metadata eliminates the need for Interface Definition Language (IDL) files, header files, or any external method of component reference.
Metadata enables .NET Framework languages to describe themselves automatically in a language-neutral manner

10. Functions or benefits of Metadata
Self-describing files
Common language runtime modules and assemblies are self-describing.
Metadata automatically provides the functionality of IDL in COM, so we can use one file for both definition and implementation.
Language interoperability and easier component-based design.
We can create an instance of any class written in any managed language without worrying about explicit marshalling or using custom interoperability code.
Attributes
Attributes are used to control in more detail how your program behaves at run time.
We can emit our own custom metadata into .NET Framework files through user-defined custom attributes.

11. External References Gokhale Sir Notes.
Metadata and Self-Describing Components
Metadata and the PE File Structure
Run-Time Use of Metadata