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To generate the thumbnail of any desired slide using Aspose.Slides for .NET:

1) Create an instance of the Presentation class. 2) Obtain the reference of any desired slide by using its ID or index. 3) Get the X and Y scaling factors based on user defined X and Y dimensions. 4) Get the thumbnail image of the referenced slide on a specified scale. 5) Save the thumbnail image in any desired image format.

//Instantiate a Presentation class that represents the presentation file
            using (PresentationEx pres = new PresentationEx("Slides Test Presentation.pptx"))
            {

                //Access the first slide
                SlideEx sld = pres.Slides[0];

                //User defined dimension
                int desiredX = 1200;
                int desiredY = 800;

                //Getting scaled value  of X and Y
                float ScaleX = (float)(1.0 / pres.SlideSize.Size.Width) * desiredX;
                float ScaleY = (float)(1.0 / pres.SlideSize.Size.Height) * desiredY;

                //Create a full scale image
                Bitmap bmp = sld.GetThumbnail(ScaleX, ScaleY);

                //Save the image to disk in JPEG format
                bmp.Save("Thumbnail2.jpg", System.Drawing.Imaging.ImageFormat.Jpeg);

            }

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Sample Code