1. .NET and .NET Core
10. Enabling Contracts
Pan Wuming
2017

2. Attribute Overview of Attributes Common Predefined Attributes
Introduction to Attributes
Applying Attributes
Common Predefined Attributes
Defining Custom Attributes
Retrieving Attribute Values

4. Introduction to Attributes
Attributes Are:
Stored with the metadata of the element
Declarative tags that convey information to the runtime
.NET Framework Provides Predefined Attributes(Intrinsic Attribute)
The runtime contains code to examine values of attributes and act on them

5. Attribute Target It’s something which attribute apply to.
Such as, Assembly, Class, Constructor, Delegate, Enum, Field, Method, Interface, Module, Parameter, Property, Return Value, Struct, All

6. Applying Attributes
Syntax: Use Square Brackets to Specify an Attribute
To Apply Multiple Attributes to an Element, You Can:
Specify multiple attributes in separate square brackets
Use a single square bracket and separate attributes with commas
[attribute(positional_parameters,
named_parameter=value, ...)]
element

7. Common Predefined Attributes
.NET Provides Many Predefined Attributes
General attributes
COM interoperability attributes
Transaction handling attributes
Visual designer component building attributes

8. Using the DllImport Attribute
With the DllImport Attribute, You Can:
Invoke unmanaged code in DLLs from a C# environment
Tag an external method to show that it resides in an unmanaged DLL
[DllImport("MyDLL.dll", EntryPoint="MessageBox")]
public static extern int MyFunction(string param1);
public class MyClass( )
{ ...
int result = MyFunction("Hello Unmanaged Code");
... }

9. Defining Custom Attributes
Defining Custom Attribute Scope
Defining an Attribute Class
Processing a Custom Attribute
Using Multiple Attributes

10. Defining Custom Attribute Target
[AttributeUsage(AttributeTargets.Class, Inherited = false)]
[Serializable]
public sealed class AttributeUsageAttribute : Attribute {
internal AttributeTargets m_attributeTarget =
AttributeTargets.All;
internal Boolean m_allowMultiple = false;
internal Boolean m_inherited = true;
public AttributeUsageAttribute(AttributeTargets validOn) {
m_attributeTarget = validOn; }
[AttributeUsage(AttributeTargets.Class
| AttributeTargets.Struct)]
public class MyAttribute: System.Attribute
{ ... }
Use the AttributeUsage Tag to Define Scope
Use the Bitwise “or” Operator (|) to Specify Multiple Elements
[AttributeUsage(AttributeTargets.Method)]
public class MyAttribute: System.Attribute
{ ... }

11. public AttributeTargets ValidOn {
get { return m_attributeTarget; } }
public Boolean AllowMultiple {
get { return m_allowMultiple; }
set { m_allowMultiple = value; }
public Boolean Inherited {
get { return m_inherited; }
set { m_inherited = value; }

12. namespace System {
[AttributeUsage(AttributeTargets.Enum, Inherited = false)]
public class FlagsAttribute : System.Attribute {
public FlagsAttribute() { … }

13. Defining an Attribute Class 1/2
Deriving an Attribute Class
All attribute classes must derive from System.Attribute, directly or indirectly
Suffix name of attribute class with “Attribute”
Components of an Attribute Class
Define a single constructor for each attribute class by using a positional parameter
Use properties to set an optional value by using a named parameter

14. Using a Custom Attribute
[BugFixAttribute(121, “David Yang", "Mar 7, 2003")]
[BugFixAttribute(121, “David Yang", "Mar 17, 2003",
Comment="Fixed off by one error")]
public class MyDisplayer {
//…………. }

15. Processing a Custom Attribute: The Compilation Process
1. Searches for the Attribute Class 2. Checks the Scope of the Attribute 3. Checks for a Constructor in the Attribute 4. Creates an Instance of the Class 5. Checks for a Named Parameter 6. Sets Field or Property to Named Parameter Value 7. Saves Current State of Attribute Object

16. Using Multiple Attributes
An Element Can Have More Than One Attribute
Define both attributes separately
An Element Can Have More Than One Instance of The Same Attribute
Use AllowMultiple = true

17. Retrieving Attribute Values
Examining Class Metadata
Querying for Attribute Information

18. Examining Class Metadata
To Query Class Metadata Information:
Use the MemberInfo class in System.Reflection
Populate a MemberInfo array by using the method System.Type.GetMembers
Create a System.Type object by using the typeof operator
Example
System.Reflection.MemberInfo[ ] memberInfoArray;
memberInfoArray = typeof(MyClass).GetMembers( );

19. Retrieving or Querying for Attribute Information
Use GetCustomAttributes to retrieve all attribute information as an array
Iterate through the array and examine the values of each element in the array
Use the IsDefined method to determine whether a particular attribute has been defined for a class

20. Contract Enabled Attribute Interact with other program
Interact with compiler
Example: Serialization
Example: Platform Invoke
Interact with CLR
Example: Caller Information

22. Common Uses for Attributes
Marking methods using the WebMethod attribute in Web services.
Describing how to marshal method parameters when interoperating with native code.
Describing the COM properties for classes, methods, and interfaces.
Calling unmanaged code using the DllImportAttribute class.
Describing your assembly in terms of title, version, description, or trademark.
Describing which members of a class to serialize for persistence.
Describing how to map between class members and XML nodes for XML serialization.
Describing the security requirements for methods.
Specifying characteristics used to enforce security.
Controlling optimizations by the just-in-time (JIT) compiler.
Obtaining information about the caller to a method.

23. Terms in this class Custom Attributes Positional parameter
Named parameter