Messages Represented as XLANGMessage

Article
02/02/2021

An XLANGMessage object represents a message instance declared with an XLANG service. This object is obtained by passing a reference to a message as a parameter in a method invocation. An XLANGPart object represents a message part contained in a message instance within an XLANG service. This object is obtained either by passing a part reference in a method invocation where the receiving parameter type is XLANGPart or by enumerating on a passed reference of XLANGMessage.

An orchestration message variable may be passed to a user component and received as an XLANGMessage object. The XLANGMessage object allows accessing the parts and accessing message properties.The user may "hold on" to an XLANGMessage and thereby extend its lifetime beyond the declared scope. Subsequently, an XLANGMessage may be returned from a method and assigned to a message variable in an orchestration.

Constructing an XLANGMessage

When constructing an XLANGMessage with a stream, the stream type must either implement IStreamFactory or be a MemoryStream. The following code sample shows how to construct an XLANGMessage:

public class FileStreamFactory : IStreamFactory  
{  
    string _fname;  
  
    public FileStreamFactory(string fname)  
    {  
        _fname = fname;  
    }  
  
    public Stream CreateStream()  
    {  
        return new FileStream  
        (  
            _fname,  
            FileMode.Open,  
            FileAccess.Read,  
            FileShare.Read  
        );  
    }  
}  
  
public static void AssignStreamFactoryToPart(XLANGMessage msg)  
{  
    IStreamFactory sf = new FileStreamFactory( @”c:\data.xml” );  
    msg[0].LoadFrom( sf );  
}

There may be times when you want to create a new message without transforming a source message. You can do this by using a variable of type System.Xml.XmlDocument and loading or otherwise constructing appropriate content. In the following example, XML is loaded from a string by using the LoadXml method of XmlDocument:

XmlVariable.LoadXml("<ns0:Root PONumber="047745351122111" xmlns:ns0="http://BTSHTTPSend.SimpleSchema"><MyChildRecord SubAttr1="Simple Attribute " /></ns0:Root>");  
XLANGMessage XmlMsg = XmlVariable;

The following example loads XML from a file by using the Load method of XmlDocument:

XmlVariable.Load("C:\MyData.xml");  
XLANGMessage XmlMsg = XmlVariable;

Note

If you want to construct larger messages, use one of the streaming methods demonstrated in the previous section or consider using the Transform shape in Orchestration Designer.

Considerations When Using XLANGMessage and XLANGPart

When using XLANGMessage and XLANGPart in user code, consider the following:

Do not pass a message part as an XLANGPart argument or return a value of type XLANGPart. You should pass XLANGPart as the type of the part. For example:
```
Message String msg;  
Class.Test(msg);  
// or you can do the following  
Messagetype mt  
{  
     String part;  
};  
Message mt msg;  
Class.Test(msg,part);  
```
You can also pass the message itself as an XLANGMessage and use the XLANGMessage subscript operators to access the part inside the function call. However, you should not put an XLANGPart in a collection whose lifetime extends past the lifetime of the function call. Instead, you should put the XLANGMessage in the collection. For example:
```
void Test(XLANGMessage xlm)
{
     try
     {
         XLANGPart xlp = xlm[0];
         string sval = (string)xlp.RetrieveAs(typeof(string));
     }
     finally
     {
         xlm.Dispose();
     }
}
```
Do not define an orchestration parameter as XLANGMessage or XLANGPart. If you want to pass a message, then use a message type parameter to pass the message. If you want to pass a part, then pass the message instead and then use the part. If you only want the part value, then use the part type to pass the part.
Do not return an XLANGMessage parameter for a method call. If you return an XLANGMessage parameter that is passed in, and then you cannot call the Dispose method on the parameter inside the method call, it intuitively violates lifetime assumptions and it also throws an exception. When passing a message through an XLANGMessage parameter to user code, you reference the message to a special context that does not normally have messages referenced to it. The lifetime of this context is the lifetime of the orchestration instance. This is because BizTalk Server does not know whether the user code will hold onto the message.

When an orchestration instance exits, any messages created in that instance are no longer valid, so the lifetime of such a collection should be less than or equal to that of the instance lifetime. However, if you want to release a message reference in a loop when the message was passed through an XLANGMessage argument that has the same lifetime as the orchestration instance, then you can call XLANGMessage.Dispose to free up the reference. Moreover, if inside the user code method, the XLANGMessage parameter is only used locally and the lifetime of the parameter is contained in that of the function call, you can also call XLANGMessage.Dispose to free up the reference to the root context and give the corresponding message back the normal lifetime behavior. For example:
```
void Test(XLANGMessage xlm)
{
     try
     {
        //XLANGMessage is only used locally
     }
     finally
     {
        xlm.Dispose();
     }
}
```
If you place the xlm in a collection, then the class itself must have a Dispose method for cleanup. For example:
```
public class A
{
     Hashtable h = new Hashtable();
     public void Test(XLANGMessage xlm)
     {
         h[xlm] = 1;
     }
     //You can have more methods here
     public void Dispose()
     {
         foreach (XLANGMessage xlm in h.Keys)
         {
             xlm.Dispose();
         }
     }
}
```
You would call A.Dispose when you are finished with the collection of XLANGMessages.

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Messages Represented as XLANGMessage

Constructing an XLANGMessage

Considerations When Using XLANGMessage and XLANGPart

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