WCF and Proxies - Requiring Ordered Delivery
(Page 6 of 6 )
In theory, the service code and the contract definition should be independent of the binding used and of its properties. The service should not care about the binding, and nothing in service code pertains to the binding used. The service should be able to work with any aspect of the configured binding. In practice, the service implementation or the contract itself may depend on ordered delivery of the messages. To enable the contract or service developer to constrain the allowed bindings, WCF defines the DeliveryRequirementsAttribute:
[AttributeUsage(AttributeTargets.Class|AttributeTargets.Interface
AllowMultiple = true)]
public sealed class DeliveryRequirementsAttribute : Attribute,...
{
public Type TargetContract
{get;set;}
public bool RequireOrderedDelivery
{get;set;}
//More members
}
TheDeliveryRequirementsattribute can be applied at the service level, affecting all endpoints of the service, or only at the endpoints that expose a particular contract. When applied at the service level, it means that requiring ordered delivery is an implementation decision. The attribute can also be used at the contract level, affecting all services that support that contract. When applied at the contract level, it means that requiring ordered delivery is a design decision. Enforcing the constraint is done at the service load time. If an endpoint has a binding that does not support reliability, or supports reliability and has reliability disabled, or has reliability enabled yet ordered delivery is disabled, loading the service will fail withInvalidOperationException.
The named pipe binding satisfies the ordered delivery constraint.
For example, to demand that all endpoints of the service, regardless of contracts, have ordered delivery enabled, apply the attribute directly on the service class:
[DeliveryRequirements(RequireOrderedDelivery = true)]
class MyService : IMyContract,IMyOtherContract
{...}
By setting theTargetContractproperty, you can demand that only endpoints of the service that support that contract be constrained to have reliable ordered delivery:
[DeliveryRequirements(TargetContract = typeof(IMyContract),
RequireOrderedDelivery = true)]
class MyService : IMyContract,IMyOtherContract
{...}
By applying theDeliveryRequirementsattribute on the contract interface, you place the constraint on all services that support it:
[DeliveryRequirements(RequireOrderedDelivery = true)]
[ServiceContract]
interface IMyContract
{...}
class MyService : IMyContract
{...}
class MyOtherService : IMyContract
{...}
The default of theRequireOrderedDeliveryisfalse, so merely applying the attribute has no effect. For example, these statements are equivalent:
[ServiceContract]
interface IMyContract
{...}
[DeliveryRequirements]
[ServiceContract]
interface IMyContract
{...}
[DeliveryRequirements(RequireOrderedDelivery = false)]
[ServiceContract]
interface IMyContract
{...}
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 This article is excerpted from chapter one of Programming WCF Services, written by Juval Lowry (O'Reilly, 2007; ISBN: 0596526997). Check it out today at your favorite bookstore. Buy this book now.
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