What Is a Shared Cell?

The first time you place a cell with Place as Shared Cell on, the shared cell definition (elements comprising the cell) is stored in the DGN file in much the same way as it is stored in the cell library. To place subsequent instances of the shared cell, the cell library does not need to be attached. In other words, a shared cell can have many instances in a DGN file but only one definition. When a shared cell instance is replaced using the Replace Cells tool, all instances of the cell are replaced.

For an unshared cell, on the other hand, the library definition is stored in the DGN file each time the cell is placed. Using shared cells can therefore be a way to reduce DGN file size. The reduction is greatest in files with cells that have a large number of component elements and/or instances.

• It is not necessary to know where the shared cell definition is in the DGN file; identifying any instance of the shared cell identifies the actual definition.
• Shared cells will not display Item Type information in the properties pane. If the shared cell is a nested cell, its children element will not be listed in the Properties pane. Hence, the associated Item Type information for the children element will also not be displayed in the Properties pane
• Any cell model with Item Type information, when placed as a shared cell, will only display the properties and Item Type information of the cell model. It will not list the elements in the model.

For example, consider a cell model Double Door placed as a graphic cell in image (a). The properties pane displays editable items and the children elements. In the image (b), the same cell model is placed as a Shared cell. The properties pane displays editable items but not the children elements. Please refer to the images below:

Cell Model Double Door

Left: Properties of a Door placed as a Graphic cell | Right: Properties of a Door placed as a Shared cell

Reasons to Use Shared Cells

The use of shared cells is recommended for these reasons:

• Shared cells are faster to place and manipulate than unshared cells. The first time a cell is placed in the DGN file, the cell library in which it is stored must be attached. If the cell is placed as a shared cell, it is not necessary to have the cell library attached to place additional instances of that cell.
• All instances of a shared cell in the DGN file are replaced when any instance of that shared cell is replaced.
• Shared cells can be associated with points on other elements, if Association Lock is on. For example, if a shared door cell is placed in a wall and associated with that wall, the door will automatically move if the wall is moved.
• Shared cells usually reduce DGN file size, thereby improving performance.

Shared Cells and Element Attributes

A shared cell has "overrides" for color, style, weight, and level that can be used in place of these attributes on the elements in the shared cell definition. There is no override for things like transparency, priority, or element class. These come from the definition of the elements in the shared cell. If you want to create a transparent shared cell you need to change the element’s definition in the shared cell.