Excerpt from Windows Communication Foundation Unleashed (WCF)

Chapter 1: Prerequisites

Authors: Craig McMurtry, Marc Mercuri, Nigel Watling, and Matt Winkler

In This Chapter

• Introduction
• Partial Types
• Generics
• Nullable Value Types
• The Lightweight Transaction Manager
• Role Providers
• References

Introduction

To properly understand and work effectively with the Windows Communication Foundation, one should be familiar with certain facilities of the 2.0 versions of the .NET Framework and the .NET common language runtime. This chapter introduces them: partial types, generics, nullable value types, the Lightweight Transaction Manager, and Role Providers. The coverage of these features is not intended to be exhaustive, but merely sufficient to clarify their use in the chapters that follow.

Partial Types

Microsoft Visual C# 2005 allows the definition of a type to be composed from multiple partial definitions distributed across any number of source code files for the same module. That option is made available via the modifier partial, which can be added to the definition of a class, an interface, or a struct. Therefore, this part of the definition of a class

public partial MyClass
{
   private string myField = null;

   public string MyProperty
   {
      get
      {
         return this.myField;
      }
   }
}

and this other part

public partial MyClass
{
   public MyClass()
   {
   }

   public void MyMethod()
   {
    this.myField = "Modified by my method.";
   }
}

can together constitute the definition of the type MyClass. This example illustrates just one use for partial types, which is to organize the behavior of a class and its data into separate source code files.

Generics

"Generics are classes, structures, interfaces, and methods that have placeholders for one or more of the types they store or use" (Microsoft 2006). Here is an example of a generic class introduced in the System.Collections.Generic namespace of the .NET Framework 2.0 Class Library:

public class List<T>

Among the methods of that class is this one:

public Add(T item)

Here, T is the placeholder for the type that an instance of the generic class System.Collections.Generic.List<T> will store. In defining an instance of the generic class, one specifies the actual type that the instance will store:

List<string> myListOfStrings = new List<string>();

Then one can use the Add() method of the generic class instance like so:

myListOfStrings.Add("Hello, World");

Evidently, generics enabled the designer of the List<T> class to define a collection of instances of the same unspecified type; in other words, to provide the template for a type-safe collection. A user of List<T> can employ it to contain instances of a type of the user's choosing, without the designer of List<T> having to know which type the user might choose. Note as well that whereas a type that is derived from a base type is meant to derive some of the functionality it requires from the base, with the remainder still having to be programmed, List<string> comes fully equipped from List<T>.

The class, System.Collections.Generic.List<T>, is referred to as a generic type definition. The placeholder, T, is referred to as a generic type parameter. Declaring

List<string> myListOfStrings;

yields System.Collections.Generic.List<string> as a constructed type, and string as a generic type argument.

Generics can have any number of generic type parameters. For example, System.Collections.Generic.Dictionary<TKey,TValue> has two.

The designer of a generic may use constraints to restrict the types that can be used as generic type arguments. This generic type definition

public class MyGenericType<T> where T: new(), IComparable

constrains the generic type arguments to types with a public, parameter-less constructor that implements the IComparable interface. This less restrictive generic type definition

public class MyGenericType<T> where T: class

merely constrains generic type arguments to reference types.

Both generic and nongeneric types can have generic methods. Here is an example of a nongeneric type with a generic method:

using System;

public class Printer
{
  public void Print<T>(T argument)
  {
    Console.WriteLine(argument.ToString());
  }

  static void Main(string[] arguments)
  {
    Printer printer = new Printer();
    printer.Print<string>("Hello, World");
    Console.WriteLine("Done");
    Console.ReadKey();
  }
}

In programming a generic, it is often necessary to determine the type of generic argument that has been substituted for a generic type parameter. This revision to the preceding example shows how one can make that determination:

public class Printer
{
  public void Print<T>(T argument)
  {
    if(typeof(T) == typeof(string))
    {
      Console.WriteLine(argument);
    }
    else
    {
      Console.WriteLine(argument.ToString());
    }
  }

  static void Main(string[] arguments)
  {
    Printer printer = new Printer();
    printer.Print<string>("Hello, World");
    Console.WriteLine("Done");
    Console.ReadKey();
  }
}

A generic interface may be implemented by a generic type or a nongeneric type. Also, both generic and nongeneric types may inherit from generic base types.

public interface IMyGenericInterface<T>
{
  void MyMethod<T>();
}

public class MyGenericImplementation<T>: IMyGenericInterface<T>
{
  public void MyMethod<T>()
  {
  }
}

public class MyGenericDescendant<T> : MyGenericImplementation<T>

{
}

public class MyNonGenericImplementation : IMyGenericInterface<string>
{
  public void MyMethod<T>()
  {
  }
}

public class MyNonGenericDescendant : MyGenericImplementation<string>
{
}

Nullable Value Types

According to the Common Language Infrastructure specification, there are two ways of representing data in .NET: by a value type or by a reference type (Ecma 2006, 18). Although instances of value types are usually allocated on a thread's stack, instances of reference types are allocated from the managed heap, and their values are the addresses of the allocated memory (Richter 2002, 134–5).

Whereas the default value of a reference type variable is null, indicating that it has yet to be assigned the address of any allocated memory, a value type variable always has a value of the type in question and can never have the value null. Therefore, although one can determine whether a reference type has been initialized by checking whether its value is null, one cannot do the same for a value type.

However, there are two common circumstances in which one would like to know whether a value has been assigned to an instance of a value type. The first is when the instance represents a value in a database. In such a case, one would like to be able to examine the instance to ascertain whether a value is indeed present in the database. The other circumstance, which is more pertinent to the subject matter of this book, is when the instance represents a data item received from some remote source. Again, one would like to determine from the instance whether a value for that data item was received.

The .NET Framework 2.0 incorporates a generic type definition that provides for cases like these in which one wants to assign null to an instance of a value type, and test whether the value of the instance is null. That generic type definition is System.Nullable<T>, which constrains the generic type arguments that may be substituted for T to value types. Instances of types constructed from System.Nullable<T> can be assigned a value of null; indeed, their values are null by default. Thus, types constructed from System.Nullable<T> may be referred to as nullable value types.

System.Nullable<T> has a property, Value, by which the value assigned to an instance of a type constructed from it can be obtained if the value of the instance is not null. Therefore, one can write

System.Nullable<int> myNullableInteger = null;
myNullableInteger = 1;
if (myNullableInteger != null)
{
  Console.WriteLine(myNullableInteger.Value);
}

The C# programming language provides an abbreviated syntax for declaring types constructed from System.Nullable<T>. That syntax allows one to abbreviate:

System.Nullable<int> myNullableInteger;

to

int? myNullableInteger;

The compiler will prevent one from attempting to assign the value of a nullable value type to an ordinary value type in this way:

int? myNullableInteger = null;
int myInteger = myNullableInteger;

It prevents one from doing so because the nullable value type could have the value null, which it actually would have in this case, and that value cannot be assigned to an ordinary value type. Although the compiler would permit this code,

int? myNullableInteger = null;
int myInteger = myNullableInteger.Value;

the second statement would cause an exception to be thrown because any attempt to access the System.Nullable<T>.Value property is an invalid operation if the type constructed from System.Nullable<T> has not been assigned a valid value of T, which has not happened in this case.

One proper way to assign the value of a nullable value type to an ordinary value type is to use the System.Nullable<T>.HasValue property to ascertain whether a valid value of T has been assigned to the nullable value type:

int? myNullableInteger = null;
if (myNullableInteger.HasValue)
{
  int myInteger = myNullableInteger.Value;
}

Another option is to use this syntax:

int? myNullableInteger = null;
int myInteger = myNullableInteger ?? -1;

by which the ordinary integer myInteger is assigned the value of the nullable integer myNullableInteger if the latter has been assigned a valid integer value; otherwise, myInteger is assigned the value of -1.

The Lightweight Transaction Manager

In computing, a transaction is a discrete activity—an activity that is completed in its entirety or not at all. A resource manager ensures that if a transaction is initiated on some resource, the resource is restored to its original state if the transaction is not fully completed. A distributed transaction is one that spans multiple resources and therefore involves more than a single resource manager. A manager for distributed transactions has been incorporated into Windows operating systems for many years. It is the Microsoft Distributed Transaction Coordinator.

.NET Framework versions 1.0 and 1.1 provided two ways of programming transactions. One way was provided by ADO.NET. That technology's abstract System.Data.Common.DbConnection class defined a BeginTransaction() method by which one could explicitly initiate a transaction controlled by the particular resource manager made accessible by the concrete implementation of DbConnection. The other way of programming a transaction was provided by Enterprise Services. It provided the System.EnterpriseServices.Transaction attribute that could be added to any subclass of System.EnterpriseServices.ServicedComponent to implicitly enlist any code executing in any of the class's methods into a transaction managed by the Microsoft Distributed Transaction Coordinator.

ADO.NET provided a way of programming transactions explicitly, whereas Enterprise Services allowed one to do it declaratively. However, in choosing between the explicit style of programming transactions offered by ADO.NET and the declarative style offered by Enterprise Services, one was also forced to choose how a transaction would be handled. With ADO.NET, transactions were handled by a single resource manager, whereas with Enterprise Services, a transaction incurred the overhead of involving the Microsoft Distributed Transaction Coordinator, regardless of whether the transaction was actually distributed.

.NET 2.0 introduced the Lightweight Transaction Manager, System.Transactions.TransactionManager. As its name implies, the Lightweight Transaction Manager has minimal overhead: "...[p]erformance benchmarking done by Microsoft with SQL Server 2005, comparing the use of a [Lightweight Transaction Manager transaction] to using a native transaction directly found no statistical differences between using the two methods" (Lowy 2005, 12). If only a single resource manager is enlisted in the transaction, the Lightweight Transaction Manager allows that resource manager to manage the transaction and the Lightweight Transaction Manager merely monitors it. However, if the Lightweight Transaction Manager detects that a second resource manager has become involved in the transaction, the Lightweight Transaction Manager has the original resource manager relinquish control of the transaction and transfers that control to the Distributed Transaction Coordinator. Transferring control of a transaction in progress to the Distributed Transaction Coordinator is referred to as promotion of the transaction.

The System.Transactions namespace allows one to program transactions using the Lightweight Transaction Manager either explicitly or implicitly. The explicit style uses the System.Transactions.CommitableTransaction class:

CommitableTransaction transaction = new CommittableTransaction();
using(SqlConnection myConnection = new SqlConnection(myConnectionString))
{
    myConnection.Open();

    myConnection.EnlistTransaction(tx);

    //Do transactional work

    //Commit the transaction:
    transaction.Close();
}

The alternative, implicit style of programming, which is preferable because it is more flexible, uses the System.Transactions.TransactionScope class:

using(TransactionScope scope = new TransactionScope)
{
    //Do transactional work:
    //...
    //Since no errors have occurred, commit the transaction:
    scope.Complete();
}

This style of programming a transaction is implicit because code that executes within the using block of the System.Transactions.TransactionScope instance is implicitly enrolled in a transaction. The Complete() method of a System.Transactions.TransactionScope instance can be called exactly once, and if it is called, then the transaction will commit.

The System.Transactions namespace also provides a means for programming one's own resource managers. However, knowing the purpose of the Lightweight Transaction Manager and the implicit style of transaction programming provided with the System.Transactions.TransactionScope class will suffice for the purpose of learning about the Windows Communication Foundation.

Role Providers

Role Providers are classes that derive from the abstract class System.Web.Security. RoleProvider. That class has the interface shown in Listing 1.1. Evidently, it defines ten simple methods for managing roles, including ascertaining whether a given user has been assigned a particular role. Role Providers, in implementing those abstract methods, will read and write a particular store of role information. For example, one of the concrete implementations of System.Web.Security.RoleProvider included in the .NET Framework 2.0 is System.Web.Security.AuthorizationStoreRoleProvider, which uses an Authorization Manager Authorization Store as its repository of role information. Another concrete implementation, System.Web.Security.SqlRoleProvider, uses a SQL Server database as its store. However, because the System.Web.Security.RoleProvider has such a simple set of methods for managing roles, if none of the Role Providers included in the .NET Framework 2.0 is suitable, one can readily provide one's own implementation to use whatever store of role information one prefers. Role Providers hide the details of how role data is stored behind a simple, standard interface for querying and updating that information. Although System.Web.Security.RoleProvider is included in the System.Web namespaces of ASP.NET, Role Providers can be used in any .NET 2.0 application.

Listing 1.1 System.Web.Security.RoleProvider

public abstract class RoleProvider : ProviderBase
{
  protected RoleProvider();

  public abstract string ApplicationName { get; set; }

  public abstract void AddUsersToRoles(
        string[] usernames, string[] roleNames);
  public abstract void CreateRole(
        string roleName);
  public abstract bool DeleteRole(
        string roleName, bool throwOnPopulatedRole);
  public abstract string[] FindUsersInRole(
        string roleName, string usernameToMatch);
  public abstract string[] GetAllRoles();
  public abstract string[] GetRolesForUser(
        string username);
  public abstract string[] GetUsersInRole(
        string roleName);
  public abstract bool IsUserInRole(
        string username, string roleName);
  public abstract void RemoveUsersFromRoles(
        string[] usernames, string[] roleNames);
  public abstract bool RoleExists(string roleName);
}

The static class, System.Web.Security.Roles, provides yet another layer of encapsulation for role management. Consider this code snippet:

if (!Roles.IsUserInRole(userName, "Administrator"))
{
 [...]
}

Here, the static System.Web.Security.Roles class is used to inquire whether a given user has been assigned to the Administrator role. What is interesting about this snippet is that the inquiry is made without an instance of a particular Role Provider having to be created first. The static System.Web.Security.Roles class hides the interaction with the Role Provider. The Role Provider it uses is whichever one is specified as being the default in the configuration of the application. Listing 1.2 is a sample configuration that identifies the role provider named MyRoleProvider, which is an instance of the System.Web.Security.AuthorizationStoreRoleProvider class, as the default role provider.

Listing 1.2 Role Provider Configuration

<configuration>
 <connectionStrings>
  <add name="AuthorizationServices"
  ´connectionString="msxml://~\App_Data\SampleStore.xml" />

 </connectionStrings>
 <system.web>
  <roleManager defaultProvider="MyRoleProvider"
   enabled="true"
   cacheRolesInCookie="true"

   cookieName=".ASPROLES"
   cookieTimeout="30"
   cookiePath="/"
   cookieRequireSSL="false"
   cookieSlidingExpiration="true"

   cookieProtection="All" >
   <providers>
    <clear />
     <add
      name="MyRoleProvider"
      type="System.Web.Security.AuthorizationStoreRoleProvider"

      connectionStringName="AuthorizationServices"
      applicationName="SampleApplication"
      cacheRefreshInterval="60"
      scopeName="" />

   </providers>
  </roleManager>
 </system.web>
</configuration>

Summary

This chapter introduced some programming tools that were new in .NET 2.0 and that are prerequisites for understanding and working effectively with the Windows Communication Foundation:

• The new partial keyword in C# allows the definitions of types to be composed from any number of parts distributed across the source code files of a single module.
• Generics are templates from which any number of fully preprogrammed classes can be created.
• Nullable value types are value types that can be assigned a value of null and checked for null values.
• The Lightweight Transaction Manager ensures that transactions are managed as efficiently as possible. An elegant new syntax has been provided for using it.
• Role Providers implement a simple, standard interface for managing the roles to which users are assigned that is independent of how the role information is stored.

References

Ecma International. 2006. ECMA-335: Common Language Infrastructure (CLI) Partitions I–VI. Geneva: Ecma.

Lowy, Juval. Introducing System.Transactions. http://www.microsoft.com/downloads/details.aspx?familyid=aac3d722-444c-4e27-8b2e-c6157ed16b15&displaylang=en. Accessed August 20, 2006.

Microsoft 2006. Overview of Generics in the .NET Framework. http://msdn2.microsoft.com/en-us/library/ms172193.aspx. Accessed August 20, 2006.

Richter, Jeffrey. 2002. Applied Microsoft .NET Framework Programming. Redmond, WA: Microsoft Press.

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