Create panel > Geometry > Extended Primitives > Object Type rollout > C-Ext button
Create menu > Extended Primitives > C-Extrusion
Tab panels > Objects tab > C-Extrusion Object
Use C-Ext to create an extruded C-shaped object.

Example of C-Ext
To create a C-Ext object:
From the Create menu, choose Extended Primitives > C-Extrusion.
Drag the mouse to define the base. (Press CTRL to constrain the base to a square.)
Release the mouse and move it vertically to define the height of the C-Ext.
Click, and then move the mouse vertically to define the thickness or width of the walls of the C-Ext.
Click to finish the C-Ext.
Corners—Draws the object from corner to corner. You can change the center location by moving the mouse.
Center—Draws the object from the center out.

Back/Side/Front Length—Specify the length of each of the three sides.
Back/Side/Front Width—Specify the width of each of the three sides.
Height—Specifies the overall height of the of the object.
Back/Side/Front Segs—Specify the number of segments for a specific side of the object.
Note: The object's dimensions (Back, Side, Front) are labeled as though it were created in the Top or Perspective viewports, and seen from the front in world space.
Width/Height Segs—Set these to specify the number of segments for the overall width and height of the object.
Generate Mapping Coords—Sets up the required coordinates for applying mapped materials to the object. Default=off.
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