This .NET class wraps the AcBrMassProps ObjectARX struct. 

The MassProperties class contains the volume, the mass (equal to the volume unless the density is specified as anything other than unity), the centroid (center of gravity), the radii of gyration (square root of the moments of inertia divided by the rigid body mass), the rigid body moments of inertia (mass times the square of the axial radius), the products of inertia (mass times the distance from the centroid to the Y,Z plane times the distance from the centroid to the X,Z plane), the principal moments at the centroid (derived from the products of inertia), and the principal axes at the centroid.

Public Structure MassProperties
End Structure

public struct MassProperties {
}

Used by BrepEntity.GetMassProperties