The .NET library, from a Xamarin project.

using System.IO;
using Aspose.Slides;

public static int CountSlides(Stream source)
{
    using (MemoryStream buffer = new MemoryStream())
    {
        source.CopyTo(buffer);
        buffer.Position = 0;

        using (Presentation presentation = new Presentation(buffer))
        {
            return presentation.Slides.Count;
        }
    }
}

using System.IO;
using System.Text;
using Aspose.Slides;

public static string ReadDeckText(Stream source)
{
    using (MemoryStream buffer = new MemoryStream())
    {
        source.CopyTo(buffer);
        buffer.Position = 0;

        IPresentationText text = PresentationFactory.Instance.GetPresentationText(
            buffer, TextExtractionArrangingMode.Unarranged);

        StringBuilder builder = new StringBuilder();
        foreach (ISlideText slide in text.SlidesText)
        {
            builder.AppendLine(slide.Text);
            builder.AppendLine(slide.NotesText);
        }

        return builder.ToString();
    }
}

using System.IO;
using Aspose.Slides;
using Aspose.Slides.Export;

public static void ReplaceText(Stream source, Stream output, string find, string replacement)
{
    using (Presentation presentation = new Presentation(source))
    {
        foreach (ISlide slide in presentation.Slides)
        {
            foreach (IShape shape in slide.Shapes)
            {
                IAutoShape autoShape = shape as IAutoShape;
                if (autoShape != null && autoShape.TextFrame != null)
                {
                    foreach (IParagraph paragraph in autoShape.TextFrame.Paragraphs)
                    {
                        foreach (IPortion portion in paragraph.Portions)
                        {
                            portion.Text = portion.Text.Replace(find, replacement);
                        }
                    }
                }
            }
        }

        presentation.Save(output, SaveFormat.Pptx);
    }
}

using System.IO;
using Aspose.Slides;

public static void ApplyLicense(Stream licenseStream)
{
    // Once per process, before the first Presentation is constructed.
    License license = new License();
    license.SetLicense(licenseStream);
}