This .NET class wraps the AcDbExtents ObjectARX class. 

It class was designed to be used with the GetSubentityGeometricExtents() function of Entity-derived classes. It embodies a box in 3D space whose edges are parallel to the axes of the WCS. This box is represented in the private data of an Extents3d object as a minimum point (MinPoint) and a maximum point (MaxPoint). At all times except immediately after an Extents3d object has been instantiated with the default constructor (see below), each of the X, Y, and Z values of MinPoint must be <= the corresponding values of MaxPoint.

Public Structure Extents3d
Inherits IFormattable
End Structure

public struct Extents3d : IFormattable {
}

The intended use of an Extents3d object is to store the "tight bounding box" of an entity or collection of entities. A typical way to compute it is to instantiate it with the default constructor, then to iterate through all the points of the geometry in question and feed them to the AddPoint() member function. This accumulates a bounding box with the smallest values encountered for X, Y, and Z in MinPoint and the largest values in MaxPoint.

Entity.GetSubentityGeometricExtents()