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Def­i­n­i­tion:

Inver­sion of Control (IoC) refers to the inver­sion of the flow of con­trol (the order in which indi­vid­ual state­ments, func­tion calls, etc. are exe­cuted) in a soft­ware. You’ll often hear the term Hol­ly­wood prin­ci­ple being men­tioned in the same breath as IoC, it sim­ply states “Don’t call us, we’ll call you” which more or less sums up the prin­ci­ples of IoC.

Pur­pose:

In tra­di­tional soft­ware design, the flow of con­trol is gov­erned by a cen­tral piece of code which often have to address mul­ti­ple con­cerns (log­ging, val­i­da­tion, etc.) and need to be aware of the imple­men­ta­tion details of its depen­den­cies. This cre­ates a very tightly cou­pled appli­ca­tion where changes in one com­po­nent have a rip­ple effect through­out the rest of the application.

Fol­low­ing the prin­ci­ples of IoC can help you achieve:

• decou­pling of exe­cu­tion of a task from imple­men­ta­tion (through the use of interfaces)
• greater sep­a­ra­tion of con­cerns (each com­po­nent only focuses on what it’s designed to do)
• more flex­i­bil­ity (imple­men­ta­tion can be eas­ily changed with­out any side effects on other components)
• more testable code (enables the use of stubs/mocks in place of con­crete classes intended for production)

Advan­tages:

• Sim­pli­fies the build­ing of spe­cific tasks.

Dis­ad­van­tages:

• Has the poten­tial to make the flow of con­trol in an appli­ca­tion more com­plex, and there­fore mak­ing it harder to follow.

Part­ing thoughts..

• Mis­us­ing or abus­ing IoC can result in Mac­a­roni code.
• IoC is not a sil­ver bul­let for all your sys­tem engi­neer­ing prob­lems, and remem­ber, “Don’t fix what’s not broken”
• When adopt­ing IoC, there is addi­tional train­ing needs for new join­ers to the team.
• Design sys­tems for flex­i­bil­ity, which allows quick adap­ta­tion to chang­ing environment/requirements
• Avoid com­pli­cat­ing sys­tem design by try­ing to be future-proof upfront, you can’t pre­dict the future!

Fur­ther readings:

.NetRocks show 362 — James Kovac Inverts our Control!

Loosen Up — Tame Your Soft­ware Depen­den­cies For More Flex­i­ble Apps (MSDN arti­cle by James Kovac)

Design Pat­tern — Inver­sion of Con­trol and Depen­dency Injec­tion (by Shiv­prasad Koirala)