create 2d barcode c# Using the ExternalDataExchange Attribute in C#.NET

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Using the ExternalDataExchange Attribute
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Although we have an interface, IMVDataService, we haven t yet done enough to provide this interface to WF and have WF actually use the interface for data communications. To do that, we need to add the ExternalDataExchange attribute. Note
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Later we ll also add attributes associated with correlation. We ll look at correlation attributes in greater detail in 17, Correlation and Local Host Communication, but for now, just think of correlation as a way to keep different data requests sorted out. I mention it here because your data communication interface is where you specify correlated data values.
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The ExternalDataExchange attribute is simply a marker that WF uses to identify interfaces suitable for local communications service use. Remember the wca.exe tool I mentioned Both it and Visual Studio use this attribute to identify this interface as one your workflow instance can use. Let s add that attribute now, along with the correlation attributes we need.
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Note
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Don t let the phrase marker attribute fool you into believing the ExternalDataExchange attribute isn t a critical component. It most certainly is. The workflow runtime looks for this attribute when attempting to make external data transfers. Without it, data transfers between workflow and host are not possible.
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Creating a workflow data communication interface
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Open Visual Studio if it isn t already open, and with the IMVDataService.cs file open for editing, add the following line of code just prior to the interface definition:
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[ExternalDataExchange]
The complete IMVDataService interface should look like the code you see in Listing 8-1. Don t worry about compiling the applications at this point. We ll need to add more code before it will compile without error.
Listing 8-1 IMVDataService.cs completed
using using using using using using System; System.Collections.Generic; System.Text; System.Workflow.Activities; System.Workflow.Runtime; System.Data;
namespace MVDataService { [ExternalDataExchange] public interface IMVDataService { void MVDataUpdate(DataSet mvData); } }
8
Calling External Methods and Workflows
Using ExternalDataEventArgs
Earlier I mentioned that, to the host application, communications from the executing workflow appear as events. The host application can t know beforehand precisely when the workflow instance will have data, and polling for data is terribly inefficient. So WF uses the asynchronous model that .NET itself uses and fires events when data is available. The host application hooks those events and reads the data. Because we want to send information to the recipient of our event, we need to create a customized event argument class. If you ve created a custom event argument class in your previous work, you probably used System.EventArgs as the base class. WF external data events, however, require a different argument base class if only to carry the instance ID of the workflow instance issuing the event. The base class we use for external data events is ExternalDataEventArgs, which itself derives from System.EventArgs, so we re on familiar ground. In addition, there are two other requirements: we must provide a base constructor that accepts the instance ID (a Guid), which in turn passes the instance ID to the base constructor, and we must mark our argument class as serializable using the Serializable attribute. Let s now build the external data event argument class we need. Creating a workflow data event argument class 1. With the MVDataService project still open in Visual Studio, locate the MVDataAvailableArgs.cs file and open it for editing. 2. You should find only the using directives and the namespace definition in the file, so after the opening brace for the namespace definition, add the following lines of code:
[Serializable] public class MVDataAvailableArgs : ExternalDataEventArgs { }
3. Finally, we need to add the required constructor to provide the base class with the workflow instance ID:
public MVDataAvailableArgs(Guid instanceId) : base(instanceId) { }
The complete event argument class is shown in Listing 8-2.
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