Take a look at this tutorial of AForge.NET Shape Checker
If your GraphicsLine object is drawing on a PictureBox or if you can convert that object to a bitmap, you can then run the code below.
You can easily try detection of quadrilaterals, this code here will actually highlight the detected objects, you do change the loop and make it do whatever you want instead:
// if you are using a PictureBox to draw the shapes, then convert it to a bitmap
Bitmap bitmap = new Bitmap(pictureBox1.Image)
// locate objects using blob counter
BlobCounter blobCounter = new BlobCounter( );
blobCounter.ProcessImage( bitmap );
Blob[] blobs = blobCounter.GetObjectsInformation( );
// create Graphics object to draw on the image and a pen
Graphics g = Graphics.FromImage( bitmap );
Pen bluePen = new Pen( Color.Blue, 2 );
// check each object and draw circle around objects, which are recognized as circles
for ( int i = 0, n = blobs.Length; i < n; i++ )
{
List<IntPoint> edgePoints = blobCounter.GetBlobsEdgePoints( blobs[i] );
List<IntPoint> corners = PointsCloud.FindQuadrilateralCorners( edgePoints );
g.DrawPolygon( bluePen, ToPointsArray( corners ) );
}
bluePen.Dispose( );
g.Dispose( );