.NET Framework Introduction:
Metadata
Ketan Bibave
Overview
• What is Metadata?
• How is Metadata?
• Structure of Metadata Tables
• Functions or benefits of Metadata
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.
How is Metadata?
• Metadata stores the following information:
– Description of the assembly.
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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.
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
• 0x06000004
– 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.
Structure of Metadata Tables
15 Type of Table
11 Defined Type
7 Module Type
4 Reference Type
4 Assembly Type
Structure of Metadata Tables: 11 Defined Type Table
Module Type
ModuleDef
0x00
TypeDef 0x02
FieldDef 0x04
Assembly Type
MethodDef
0x06
ParamDef
0x08
PropertyDef
0x17
EventDef
0x14
AssemblyDef
0x20
FileDef
0x26
ExportedTypeDef0x27
ManifestResourceDef
0x28
Structure of Metadata Tables: 4 Reference Type Table
Reference Type
AssemblyRef
0x23
ModuleRef
0x1
TypeRef 0x01
MemberRef
0x0a
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
Functions or benefits of Metadata
• Self-describing files
– Common language runtime modules and assemblies are selfdescribing.
– 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.
External References
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Gokhale Sir Notes.
Metadata and Self-Describing Components
Metadata and the PE File Structure
Run-Time Use of Metadata