code to generate barcode in vb.net Part IV in C#

Encoder UPC-A Supplement 5 in C# Part IV

Part IV
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External Data Communication
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HandleExternalEvent activities. We used the wca.exe tool to create custom activities for us based on our interface. (We could have used the CallExternalEvent and HandleExternalEvent activities directly, providing each with the interface and method or event to process, but creating custom activities highlights their use in our workflows.) With the interface in hand, we then created a local service that we plugged in to the workflow runtime to manage our local communications needs. The local service consisted of a data connector and a service class. When the application needed to send data to the workflow, it requested the service from the workflow runtime and then raised the events supported by the interface. Your workflow instance would then handle those events, assuming you dropped the event handler into the workflow and invoked the event at an appropriate time (for example, when the workflow anticipated the event and was ready and waiting with the correct event handler). The workflow, on the other hand, had no such need to query the workflow runtime for the local communication service. By dropping instances of CallExternalMethod in your workflow s processing path, the host would automatically be told of the data s arrival again assuming the host application hooked an event handler into the local communication service for the received data event. The workflow runtime keeps track of workflow instances and their association with the local communication service and therefore the host application.
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Consider that last paragraph again. The workflow instance didn t need to rummage around to find the service that communicates with the host application. Yet the host application did need to query for the local communication service. Although this, in part, is due to the nature of host interaction with the workflow runtime, the process also underscores the one-to-many relationship between the host application and workflow instances. The host application needs to identify which workflow instance it wants to communicate with because there might be many to choose from. However, a workflow instance has no such choice there is only one host application. I m not saying the need for correlated data flow drove the WF team to architect the host application s access to the local communication service in this way. The host always queries the workflow runtime for services in this fashion, and the local communication service is just one service you might want to access. However, the reverse is certainly true. The workflow is bound to the local communication service with no regard for host application identification, and this is a direct result of an architecture designed around having many workflow instances in one host application. There can be no more than one host application, so there is no need to identify it. Therefore, the workflow runtime provides the workflow instance with the local communication service, and the workflow instance calls external methods at will.
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17
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So is it enough for the host to use the workflow instance identifier as the way to correlate data flow That is, if you keep track of a workflow instance and try to send data back and forth to and from that workflow, wouldn t merely having the workflow s instance ID be enough to uniquely identify the workflow and the data Yes, if you had but a single data flow. But it s possible to have multiple data paths into and out of your workflow. For this reason, correlation was born. When you use correlated workflow communication, in the end the workflow runtime creates a storage container for bits of information necessary to identify the workflow and data in question. This correlation token is consulted when the host and workflow pass data back and forth. If the correlation token indicates that both sides of the conversation are in sync, meaning the correct workflow instance and bound activity are communicating the correct piece of data, communication can proceed. However, if the correlation token indicates a problem, the workflow runtime does not allow the data communication to proceed and throws an exception. Problems might include using an incorrect workflow instance, communicating the wrong data, calling an activity bound to a different correlation token, or trying to send data without first creating the correlation token. The correlation token is maintained by the CorrelationToken class. When you drop copies of the CallExternalMethod or HandleExternalEvent activity into your workflow, and if correlation is involved, you need to assign a correlation token. Correlation tokens are shared by name, so by assigning a correlation token with the same name to more than one correlated activity, you effectively bind those activities together from a data-conversation perspective. The token s name is nothing more than a string, the value of which is meaningless. It only matters that activities that share a correlation token share it by identifying the token using the same name. A good question now is, Why didn t correlation tokens come into play earlier in the book After all, we certainly have used both CallExternalMethod and HandleExternalEvent activities in previous work. The answer is we chose not to invoke correlation. Correlation isn t required in all cases, and this was true for every workflow you created up until this chapter. When you have one-to-one mapping between the application and workflow instance, correlation is unnecessary overhead, and happily you can omit it and enjoy slightly increased performance. Even when you have multiple workflow instances working with a single host application, you can work without correlation. However, when you use correlation, WF prevents you from inadvertently mixing up data, and in many cases, this is a very desirable feature. To activate the correlation infrastructure, you use specific WF-based attributes when you create your host communication interface. The good news is the process to perform the host communication doesn t change by much. The effect the change has on your workflow can be dramatic, however.
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