Area of rectangle (area=l*b)

using System;
public class AreaOfRectangle
{
    public static void Main()
    {
        int Length = 15;
        int Breadth = 6;
        int Area = Length * Breadth;
        Console.WriteLine($"Area of Length {Length} and Breadth {Breadth} Rectangle is {Area}");
        Console.ReadKey();
    }
} 

I assume you want to calculate area of rectangle using c#.

Please see the below code.

using System;
public class AreaOfRectangle
{
    public static void Main()
    {
        int Length = 15;
        int Breadth = 6;
        int Area = Length * Breadth;
        Console.WriteLine($"Area of Length {Length} and Breadth {Breadth} Rectangle is {Area}");
        Console.ReadKey();
    }
} 

You can use LEADTOOLS Imaging Pro SDK technology in your application.
https://www.leadtools.com/sdk/products/imaging-pro
You can use GetAreaInPixels method inside the AnnRectangleObject class. This method is used to return the area bounded inside an AnnRectangleObject in pixels.

Here is some code:

// Load the image
using (RasterCodecs _codecs = new RasterCodecs())
using (RasterImage _image = _codecs.Load(@"FILE PATH TO SOURCE IMAGE"))
{
    // Create the annotation container to store the annotation objects
    AnnContainer _container = new AnnContainer();
    _container.Mapper.MapResolutions(_image.XResolution, _image.YResolution, _image.XResolution, _image.YResolution);
    _container.Size = _container.Mapper.SizeToContainerCoordinates(_image.ImageSize.ToLeadSizeD());

    // Annotation unit is 1/720 of an inch
    double inch = 720.0;

    // Add 1st rectangle as annotation
    AnnRectangleObject _annRect = new AnnRectangleObject();
    _annRect.Rect = new LeadRectD(1 * inch, 2 * inch, 1 * inch, 3 * inch);
    _annRect.Stroke = AnnStroke.Create(AnnSolidColorBrush.Create("Green"), LeadLengthD.Create(1));
    _container.Children.Add(_annRect);

    // Get the area of the rectangle
    double rectArea = _annRect.GetAreaInPixels(_container.Mapper);
}