[Microstation CE u13] ElementGraphicsProcessor example in C#

Question

Hi all, 
 I am trying to extract all of the elements contained within a paramteric cell, and it was suggested that I use the ElementGraphicsProcessor abstract class to achieve this. The objective is to to return a ll graphical elements from the cell instance in their true location (with the same geometry), along with their associated symbology. Essentially this would have a similar outcome as dropping the cell however the typical workflow for dropping an element programmatically doesn't seem to work on parametric cells unfortunately. Does anyone have any examples or documentation that could enlighten me on how to achieve this? 
 Thanks! 
 Ed

Jan Šlegr · Accepted Answer

Hi Ed,

Edward Ashbolt said:
Please bear in mind I am very new to all of this

New to what? One thing is start with MicroStation NET API, which can be confusing, but now much better knowledge base (examples, blogs, discussion) is available than when CE was released. But sometimes my feeling is that for people the problem is not in new API, but in weak knowledge of basic OOP concepts (SOLID etc.).

Edward Ashbolt said:
but how do I then enumerate the results / pass this as a variable to DraftingElementSchema.

In my opinion dependency injection is the solution here.

Edward Ashbolt said:
Do I need to add a List to collect the elements within the processor?

It's up to you what is your intention and also what software architecture do you prefer (because it can be implemented in several ways). Also, do not forget that Element is IDisposable, so you have to ensure it will be disposed explicitly.

Edward Ashbolt said:
It throws an error on line 24, most likely due to null value for template.

Absolutely not! When you say "error", it's like silence. What error? Is an exception thrown? When yes, what exception?

Edward Ashbolt said:
Updated code:

There are some ugliness:

What fashion (because I do not know about any reason) I have seen in many shared snippets to pollute code with full classes names (including namespaces)?
I do not know GC API, so I am not sure what is the best way, but to use ModelRef to obtain element plus using explicit casting (which potentially can ends with casting exception) looks not very elegant.

This should work (at least MicroStation part), but be aware it's a concept, the structure is not fine (especially how elements are disposed):

public NodeUpdateResult DropParametricCell
    (
        NodeUpdateContext updateContext,
        [Replicatable, DgnModelProvider] object ParametricCell
    )
    {
        try
        {
            Feature feat = ParametricCell as Feature;
            long id = feat.Element.ID;

            DgnModel model = Session.Instance.GetActiveDgnModel();
            Element element = dgnModel.FindElementById(new ElementId(ref id));
            
            ParametricCellProcessor processor = new ParametricCellProcessor();


            if (element is ParametricCellElement instance)
            {
                MessageCenter.Instance.StatusMessage = $"Parametric cell: {instance.CellDefinition.CellName}";

                List<Element> extracted = new List<Element>();

                try
                {
                    ParametricCellProcessor processor = new ParametricCellProcessor(extracted, model);
                    ElementGraphicsOutput.Process(instance, processor);

                    foreach (Element el in extracted)
                    {
                        el.AddToModel();
                    }
                }
                finally
                {
                    if (null != extracted)
                    {
                        foreach (Element el in extracted)
                        {
                            el.Dispose();
                        }
                    }
                }
            }
        }
        catch (Exception ex)
        {
            return new NodeUpdateResult.TechniqueException(ex);
        }

        return NodeUpdateResult.Success;
        
    }
} // class

internal class ParametricCellProcessor : ElementGraphicsProcessor
{
    private readonly IList<Element> elements;
    private readonly DgnModel dgnModel;
    private DTransform3d trans;

    private ParametricCellProcessor()
    {
    }

    public ParametricCellProcessor(IList<Element> elements, DgnModel dgnModel)
    {
        this.elements = elements ?? throw new ArgumentNullException(nameof(elements));
        this.dgnModel = dgnModel ?? throw new ArgumentNullException(nameof(dgnModel));
    }

    public override void AnnounceTransform(DTransform3d trans)
    {
        this.trans = trans;
    }

    public override bool ProcessAsBody(bool isCurved) => false;

    public override bool ProcessAsFacets(bool isPolyface) => false;

    public override BentleyStatus ProcessCurveVector(CurveVector curves, bool isFilled) => BentleyStatus.Error;

    public override BentleyStatus ProcessCurvePrimitive(CurvePrimitive curve, bool isClosed, bool isFilled)
    {
        Element element = DraftingElementSchema.ToElement(this.dgnModel, curve, null);
        element.ApplyTransform(new TransformInfo(this.trans));
        this.elements.Add(element);
        return BentleyStatus.Success;
    }
}

Regards,

Jan

Bentley Accredited Developer: iTwin Platform - Associate
Labyrinth Technology | dev.notes() | cad.point

Jan Šlegr · Answer

Hi Edward, 
 Edward Ashbolt said: and it was suggested that I use the ElementGraphicsProcessor abstract class to achieve this 
 Er ... no, in fact. I recommend to use ElementGraphicsProcessor to obtain geometry, not elements! It's important to distinguish between these two different information ;-) 
 Edward Ashbolt said: The objective is to to return a ll graphical elements from the cell instance in their true location (with the same geometry) 
 In general, some objects allow to access internal elements, some not. There is even a method called ExposeChildren to determine whether this functionality is supported. My feeling is that general concept of new API is to do not expose internal elements (as elements), because it's implementation / persistence detail, but to prefer to use the object geometry everywhere it's possible. 
 Whereas e.g. plain cell is the example of the element exposing children, parametric cell is black bock and it's not possible to access original elements (at lest in the current API version). What is not clear whether it's "by design" or it's just "not implemented yet" feature, because constraints-based modeling is still under development and new features are added. 
 Edward Ashbolt said: the typical workflow for dropping an element programmatically doesn't seem to work on parametric cells unfortunately 
 I think the drop tool does not work well with parametric cells in general, not only in API. 
 Edward Ashbolt said: Does anyone have any examples or documentation that could enlighten me on how to achieve this? 
 There is, the processor is used in some example delivered with SDK and when you will search this forum, you will find discussion about different aspects of this (great and clever!) object ... also because it seems in some situations the wrapper around native processor does not work as expected. 
 The code can be similar to this one: 
 internal class Demo
 {
 public void IterateAllElements()
 {
 ParametricCellProcessor processor = new ParametricCellProcessor();

 DgnModel model = Session.Instance.GetActiveDgnModel();

 ModelElementsCollection elements = model.GetGraphicElements();

 foreach (Element element in elements)
 {
 if (element is ParametricCellElement instance)
 {
 MessageCenter.Instance.StatusMessage = $"Parametric cell: {instance.CellDefinition.CellName}";
 ElementGraphicsOutput.Process(instance, processor);
 }
 }
 }
 }

 internal class ParametricCellProcessor : ElementGraphicsProcessor
 {
 public override bool ProcessAsBody(bool isCurved)
 {
 return false;
 }

 public override bool ProcessAsFacets(bool isPolyface)
 {
 return false;
 }

 public override BentleyStatus ProcessCurveVector(CurveVector curves, bool isFilled)
 {
 return BentleyStatus.Error;
 }

 public override BentleyStatus ProcessCurvePrimitive(CurvePrimitive curve, bool isClosed, bool isFilled)
 {
 MessageCenter.Instance.StatusMessage = $"{curve.GetCurvePrimitiveType()}";
 return BentleyStatus.Success;
 }

 public override bool WantClipping()
 {
 return false;
 }
 } 
 I recommend (in fact, I think you have to) to read ElementGraphicsProcessor description in native MicroStatioAPI doc, because it's crucial to understand what "bool returning" methods should be overridden to obtain what result. 
 When you are interested in symbology (or other "not geometry info), override proper AnnounceXYZ method. 
 With regards, 
 Jan

[Microstation CE u13] ElementGraphicsProcessor example in C#

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