.NET metadata, in the Microsoft .NET framework, refers to certain data structures embedded within the Common Intermediate Language
Common Intermediate Language
Common Intermediate Language is the lowest-level human-readable programming language defined by the Common Language Infrastructure specification and is used by the .NET Framework and Mono...
code that describes the high-level structure of the code. Metadata describes all classes
Class (computer science)
In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...
and class members that are defined in the assembly, and the classes and class members
Method (computer science)
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
that the current assembly will call from another assembly. The metadata for a method contains the complete description of the method, including the class (and the assembly that contains the class), the return type
Return type
In computer programming, the return type defines and constrains the data type of value returned from a method or function...
In computer programming, a parameter is a special kind of variable, used in a subroutine to refer to one of the pieces of data provided as input to the subroutine. These pieces of data are called arguments...
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
Common Intermediate Language is the lowest-level human-readable programming language defined by the Common Language Infrastructure specification and is used by the .NET Framework and Mono...
The Common Language Runtime is the virtual machine component of Microsoft's .NET framework and is responsible for managing the execution of .NET programs. In a process known as just-in-time compilation, the CLR compiles the intermediate language code known as CIL into the machine instructions...
executes CIL it will check to make sure that the metadata of the called method is the same as the metadata that is stored in the calling method. This ensures that a method can only be called with exactly the right number of parameters and exactly the right parameter types.
Attributes
Developers can add metadata to their code through attributes. There are two types of attributes, custom and pseudo custom attributes, and to the developer these have the same syntax. Attributes in code are messages to the compiler to generate metadata. In CIL, metadata such as inheritance modifiers, scope modifiers, and almost anything that isn't either opcodes or streams, are also referred to as attributes.
A custom attribute is a regular class that inherits
Inheritance
Inheritance is the practice of passing on property, titles, debts, rights and obligations upon the death of an individual. It has long played an important role in human societies...
from the Attribute class. A custom attribute can be used on any method, property, class or entire assembly with the syntax: [AttributeName(optional parameter, optional name=value pairs)] as in:
[Custom]
[Custom(1)]
[Custom(1, Comment="yes")]
Custom attributes are used by the .NET Framework extensively. Windows Communication Framework uses attributes to define service contracts, ASP.NET
ASP.NET
ASP.NET is a Web application framework developed and marketed by Microsoft to allow programmers to build dynamic Web sites, Web applications and Web services. It was first released in January 2002 with version 1.0 of the .NET Framework, and is the successor to Microsoft's Active Server Pages ...
A Web service is a method of communication between two electronic devices over the web.The W3C defines a "Web service" as "a software system designed to support interoperable machine-to-machine interaction over a network". It has an interface described in a machine-processable format...
Linq is a word-based card game from Endless Games, introduced at the American International Toy Fair in 2005.Game play requires at least four players, two of whom are dealt cards with the same word, while the others receive blanks. The goal is to gain points by correctly naming the players with...
uses them to define the mapping of classes to the underlying relational schema, Visual Studio uses them to group together properties of an object, the class developer indicates the category for the object's class by applying the [Category] custom attribute. Custom attributes are interpreted by application code and not the CLR.When the compiler sees a custom attribute it will generate custom metadata that is not recognised by the CLR. The developer has to provide code to read the metadata and act on it. As an example, the attribute shown in the example can be handled by the code:
class CustomAttribute : Attribute
{
private int paramNumber = 0;
private string comment = "";
public CustomAttribute { }
public CustomAttribute(int num) { paramNumber = num; }
public String Comment
{
set { comment = value; }
}
}
The name of the class is mapped to the attribute name. The Visual C# compiler automatically adds the string "Attribute" at the end of any attribute name. Consequently every attribute class name should end with this string, but it is legal to define an attribute without the Attribute-suffix. When affixing an attribute to an item, the compiler will look for both the literal name and the name with Attribute added to the end, i.e. if you were to write [Custom] the compiler would look for both Custom and CustomAttribute. If both exist, the compiler fails. The attribute can be prefixed with "@" if you don't want to risk ambiguity, so writing [@Custom] will not match CustomAttribute. Using the attribute invokes the constructor of the class. Overloaded constructors are supported. Name-Value pairs are mapped to properties, the name denotes the name of the property and the value supplied is set by the property.
Sometimes there is ambiguity concerning to what you are affixing the attribute. Consider the following code:
[Orange]
public int ExampleMethod(string input)
{
//method body goes here
}
What has been marked as orange? Is it the ExampleMethod, its return value, or perhaps the entire assembly? In this case, the compiler will default, and treat the attribute as being affixed to the method. If this is not what was intended, or if the author wishes to clarify their code, an attribute target may be specified. Writing [return: Orange] will mark the return value as orange, [assembly: Orange] will mark the entire assembly. The valid targets are assembly, field, event, method, module, param, property, return and type.
A pseudo-custom attribute is used just like regular custom attributes but they do not have a custom handler; rather the compiler has intrinsic awareness of the attributes and handles the code marked with such attributes differently. Attributes such as Serializable and Obsolete are implemented as pseudo-custom attributes. Pseudo-custom attributes should never be used by ILASM, as it has adequate syntax to describe the metadata.
Metadata storage
Assemblies contain tables of metadata. These tables are described by the CIL specification. The metadata tables will have zero or more entries and the position of an entry determines its index. When CIL code uses metadata it does so through a metadata token. This is a 32-bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
value where the top 8 bits identify the appropriate metadata table, and the remaining 24 bits give the index of the metadata in the table. The Framework SDK contains a sample called metainfo that will list the metadata tables in an assembly, however, this information is rarely of use to a developer. Metadata in an assembly may be viewed using the ILDASM tool provided by the .NET Framework SDK.
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...
used to read .NET metadata. The reflection API provides a logical view of metadata rather than the literal view provided by tools like metainfo. Reflection in version 1.1 of the .NET framework can be used to inspect the descriptions of classes and their members, and invoke methods. However, it does not allow runtime access to the CIL for a method. Version 2.0 of the framework allows the CIL for a method to be obtained.
Other metadata tools
Besides the System.Reflection namespace, other tools are also available that can be used to handle metadata. The Microsoft .NET Framework ships a CLR metadata manipulation library that is implemented in native code. Third party tools to retrieve and manipulate metadata include PostSharp and Mono Cecil can also be used.
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