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OpenXML Presentation:

In below functionality by default a slide is added to presentation.Here we are adding new slide at index 2 having some text in it.

class Program
    {
        static void Main(string[] args)
        {
            InsertNewSlide("Sample.pptx", 1, "My new slide");
        }
        // Insert a slide into the specified presentation.
        public static void InsertNewSlide(string presentationFile, int position, string slideTitle)
        {
            // Open the source document as read/write. 
            using (PresentationDocument presentationDocument = PresentationDocument.Open(presentationFile, true))
            {
                // Pass the source document and the position and title of the slide to be inserted to the next method.
                InsertNewSlide(presentationDocument, position, slideTitle);
            }
        }

        // Insert the specified slide into the presentation at the specified position.
        public static void InsertNewSlide(PresentationDocument presentationDocument, int position, string slideTitle)
        {

            if (presentationDocument == null)
            {
                throw new ArgumentNullException("presentationDocument");
            }

            if (slideTitle == null)
            {
                throw new ArgumentNullException("slideTitle");
            }

            PresentationPart presentationPart = presentationDocument.PresentationPart;

            // Verify that the presentation is not empty.
            if (presentationPart == null)
            {
                throw new InvalidOperationException("The presentation document is empty.");
            }

            // Declare and instantiate a new slide.
            Slide slide = new Slide(new CommonSlideData(new ShapeTree()));
            uint drawingObjectId = 1;

            // Construct the slide content.            
            // Specify the non-visual properties of the new slide.
            NonVisualGroupShapeProperties nonVisualProperties = slide.CommonSlideData.ShapeTree.AppendChild(new NonVisualGroupShapeProperties());
            nonVisualProperties.NonVisualDrawingProperties = new NonVisualDrawingProperties() { Id = 1, Name = "" };
            nonVisualProperties.NonVisualGroupShapeDrawingProperties = new NonVisualGroupShapeDrawingProperties();
            nonVisualProperties.ApplicationNonVisualDrawingProperties = new ApplicationNonVisualDrawingProperties();

            // Specify the group shape properties of the new slide.
            slide.CommonSlideData.ShapeTree.AppendChild(new GroupShapeProperties());

            // Declare and instantiate the title shape of the new slide.
            Shape titleShape = slide.CommonSlideData.ShapeTree.AppendChild(new Shape());

            drawingObjectId++;

            // Specify the required shape properties for the title shape. 
            titleShape.NonVisualShapeProperties = new NonVisualShapeProperties
                (new NonVisualDrawingProperties() { Id = drawingObjectId, Name = "Title" },
                new NonVisualShapeDrawingProperties(new Drawing.ShapeLocks() { NoGrouping = true }),
                new ApplicationNonVisualDrawingProperties(new PlaceholderShape() { Type = PlaceholderValues.Title }));
            titleShape.ShapeProperties = new ShapeProperties();

            // Specify the text of the title shape.
            titleShape.TextBody = new TextBody(new Drawing.BodyProperties(),
                    new Drawing.ListStyle(),
                    new Drawing.Paragraph(new Drawing.Run(new Drawing.Text() { Text = slideTitle })));

            // Declare and instantiate the body shape of the new slide.
            Shape bodyShape = slide.CommonSlideData.ShapeTree.AppendChild(new Shape());
            drawingObjectId++;

            // Specify the required shape properties for the body shape.
            bodyShape.NonVisualShapeProperties = new NonVisualShapeProperties(new NonVisualDrawingProperties() { Id = drawingObjectId, Name = "Content Placeholder" },
                    new NonVisualShapeDrawingProperties(new Drawing.ShapeLocks() { NoGrouping = true }),
                    new ApplicationNonVisualDrawingProperties(new PlaceholderShape() { Index = 1 }));
            bodyShape.ShapeProperties = new ShapeProperties();

            // Specify the text of the body shape.
            bodyShape.TextBody = new TextBody(new Drawing.BodyProperties(),
                    new Drawing.ListStyle(),
                    new Drawing.Paragraph());

            // Create the slide part for the new slide.
            SlidePart slidePart = presentationPart.AddNewPart<SlidePart>();

            // Save the new slide part.
            slide.Save(slidePart);

            // Modify the slide ID list in the presentation part.
            // The slide ID list should not be null.
            SlideIdList slideIdList = presentationPart.Presentation.SlideIdList;

            // Find the highest slide ID in the current list.
            uint maxSlideId = 1;
            SlideId prevSlideId = null;

            foreach (SlideId slideId in slideIdList.ChildElements)
            {
                if (slideId.Id > maxSlideId)
                {
                    maxSlideId = slideId.Id;
                }

                position--;
                if (position == 0)
                {
                    prevSlideId = slideId;
                }

            }

            maxSlideId++;

            // Get the ID of the previous slide.
            SlidePart lastSlidePart;

            if (prevSlideId != null)
            {
                lastSlidePart = (SlidePart)presentationPart.GetPartById(prevSlideId.RelationshipId);
            }
            else
            {
                lastSlidePart = (SlidePart)presentationPart.GetPartById(((SlideId)(slideIdList.ChildElements[0])).RelationshipId);
            }

            // Use the same slide layout as that of the previous slide.
            if (null != lastSlidePart.SlideLayoutPart)
            {
                slidePart.AddPart(lastSlidePart.SlideLayoutPart);
            }

            // Insert the new slide into the slide list after the previous slide.
            SlideId newSlideId = slideIdList.InsertAfter(new SlideId(), prevSlideId);
            newSlideId.Id = maxSlideId;
            newSlideId.RelationshipId = presentationPart.GetIdOfPart(slidePart);

            // Save the modified presentation.
            presentationPart.Presentation.Save();
        }

    }

Aspose.Slides:

Each PowerPoint presentation file contains one Main Master slide and other Normal slides. It means that a presentation file contains at least one or more slides. It is important to know that presentation files without slides are not supported by Aspose.Slides for .NET. Each slide has specific position and a unique Id. The slide Id can range from 0 to 255 for master slides and from 256 to 65535 for normal slides.

Aspose.Slides for .NET allows developers to add empty slides to the presentations using the AddEmptySlide method exposed by Presentation object.To add an empty slide in the presentation, please follow the steps below:

Create an instance of Presentation class
Call the AddEmptySlide method exposed by Presentation object
Do some work with the newly added empty slide
Add another slide and insert text on it.

Finally, write the PPT file using the Write method exposed by Presentation object

class Program
    {
        static void Main(string[] args)
        {
            //Instantiate PresentationEx class that represents the PPT file
            Presentation pres = new Presentation();

            //Blank slide is added by default, when you create
            //presentation from default constructor
            //Adding an empty slide to the presentation and getting the reference of
            //that empty slide
            Slide slide = pres.AddEmptySlide();

            slide = pres.GetSlideByPosition(2);

            //Get the font index for Arial 
            //It is always 0 if you create presentation from
            //default constructor
            int arialFontIndex = 0;

            //Add a textbox
            //To add it, we will first add a rectangle
            Shape shp = slide.Shapes.AddRectangle(1200, 800, 3200, 370);

            //Hide its line
            shp.LineFormat.ShowLines = false;

            //Then add a textframe inside it
            TextFrame tf = shp.AddTextFrame("");

            //Set a text
            tf.Text = "Text added dynamically";
            Portion port = tf.Paragraphs[0].Portions[0];
            port.FontIndex = arialFontIndex;
            port.FontBold = true;
            port.FontHeight = 32;

            //Write the output to disk
            pres.Write("outAspose.ppt");

        }
    }

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Sample Code