Intelligent code completion

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Intelligent code completion[1] is a context-aware code completion feature in some programming environments that speeds up the process of coding applications by reducing typos and other common mistakes. Attempts to do this are usually done through auto completion popups when typing, querying parameters of functions, query hints related to syntax errors, etc. Intelligent code completion and related tools serve as documentation and disambiguation for variable names, functions and methods using reflection.[2]

Intelligent code completion appears in many program environments,[3][4] example implementations being Atom's autocomplete + and Visual Studio's IntelliSense. The term was originally popularized as "picklist" and some implementations still refer to it as such.[5]


Similar to other autocompletion systems, intelligent code completion is a convenient way to access descriptions of functions, particularly their parameter lists. It speeds up software development by reducing the amount of name memorization needed and keyboard input required. It also allows for less reference to external documentation as interactive documentation on many symbols (i.e. variables and functions) in the active scope appears dynamically in the form of tooltips while programming.[6]

Intelligent code completion works using an automatically generated in-memory database of classes, variable names, and other constructs that the application being edited defines or references. The "classic" implementation of IntelliSense works by detecting marker characters such as periods, or other separator characters, depending on the language. When the user types one of these characters immediately after the name of an entity having one or more accessible members (such as contained variables or functions), IntelliSense suggests matches in a pop-up window. The user can either accept the suggestion by typing a statement completion character (Tab ↹ or ↵ Enter)—or a language-specific marker (such as the semicolon for C++), or continue typing the name. Over time, IntelliSense determines which variable or function the user most likely wants.

The feature also lets the user select from a number of overloaded functions in languages that support object oriented programming. IntelliSense also displays a short description of a function in the pop-up window—depending on the amount of documentation in the function's source code.


Several of the original components of an intelligent code completion system were designed and invented by Gail Kaiser, a professor at Columbia University's Department of Computer Science.[7] She created many of the basic intelligent code completion techniques in most if not all modern programming environments, starting in 1988, while working at Columbia.[8]

Research extends back to 1957, including spelling checkers for bitmap images of cursive writing and special applications to find records in databases in spite of incorrect entries. In 1961, Les Earnest, who headed the research on this budding technology, saw it necessary to include the first spell checker that accessed a list of 10,000 acceptable words.[9] Ralph Gorin, a graduate student under Earnest at the time, created the first true spelling checker program written as an applications program (rather than research) for general English text: Spell for the DEC PDP-10 at Stanford University's Artificial Intelligence Laboratory, in February 1971.[10] Gorin wrote SPELL in assembly language, for faster action; he made the first spelling corrector by searching the word list for plausible correct spellings that differ by a single letter or adjacent letter transpositions and presenting them to the user. Gorin made SPELL publicly accessible, as was done with most SAIL (Stanford Artificial Intelligence Laboratory) programs, and it soon spread around the world via the new ARPAnet, about ten years before personal computers came into general use.[11] Spell, its algorithms and data structures inspired the Unix ispell program.

Support in editors and IDEs[edit]

Visual Studio[edit]

IntelliSense is Microsoft's implementation of code completion, best known in Visual Studio. It was first introduced as a feature of a mainstream Microsoft product in 1996 building on many already invented concepts of code completion and syntax checking, with the Visual Basic 5.0 Control Creation Edition, which was essentially a publicly available prototype for Visual Basic 5.0. Initially, Visual Basic IDE was the primary "test bed" for the technology, but IntelliSense was incorporated into Visual FoxPro and Visual C++ in the Visual Studio 97 timeframe (one revision after first seen in Visual Basic). Because it was based on the introspection capabilities of COM, the Visual Basic versions of IntelliSense were always more robust and complete than the 5.0 and 6.0 (97 and 98 in the Visual Studio naming sequence) versions of Visual C++, which did not have the benefit of being entirely based on COM. These shortcomings (criticized by many VC++ developers since the 97 release) have been largely corrected in the .NET product lines. For example, one of the most requested capabilities missing from the pre-.NET products was support for templates, which is now fully implemented.[12]

IntelliSense has entered a new phase of development with the unified Visual Studio.NET environment first released in 2001, augmented by the more powerful introspection and code documentation capabilities provided by the .NET framework. IntelliSense is now supported by the Visual Studio editors for C++, C#, J#, Visual Basic, XML, HTML and XSLT among others. As of Visual Studio 2005, IntelliSense is now activated by default when the user begins to type, instead of requiring marker characters (though this behavior can be turned off). The IDE has the capability of inferring a greater amount of context based on what the developer is typing, to the point that basic language constructs such as for and while are also included in the choice list. In 2017 Microsoft announced IntelliCode,[13] which uses machine learning to infer exactly which language or library feature is likely to be intended at every keystroke. Initially available as an extension for C# only, it is expected to be built in to future releases of Visual Studio.

Other Microsoft products that incorporate IntelliSense include Expression Web, FrontPage 2003, Small Basic, the Visual Basic for Applications IDEs in the Microsoft Office products, Visual Studio Code and many others. SQL Server 2008 Management Studio has autocomplete for the SQL syntax.


The Eclipse IDE has code completion tools that come packaged with the program.[14][15] It includes notable support for Java, C++, and JavaScript code authoring. The Code Recommenders Eclipse project used to provide powerful intelligent completion,[16] but due to lack of resources, was dropped in Eclipse 2018–12, and then archived in July 2019.[17][18][19]


Vim Intellisense[20] is an advanced code completion system for the Vim editor.


Assume a C++ application being edited in Visual Studio has a class Foo with some member functions:

class Foo {
    void bar();
    void foo_bar(char c, int n);

When the developer references this class in source code, e.g.:

Foo foo;

as soon as the user types the period after foo, IntelliSense automatically lists all the available member functions (i.e. bar() and foo_bar()) and all the available member attributes (private and protected members can be identified by a padlock picture beside their names). The user can then select one by using the arrow keys and hitting a completion character when the correct member function is highlighted. When available, IntelliSense displays a short description of the member function as given in the source code documentation.

IntelliSense goes further by indicating the required parameters in another pop-up window as the user fills in the parameters. As the user types a variable name, the feature also makes suggestions to complete the variable as they are typed. IntelliSense continues to show parameters, highlighting the pertinent one, as the user types.

The user can "force" IntelliSense to show its pop-up list without context by using Ctrl+J or Ctrl+Space. In Visual Studio this displays the entire application domain object model available to the developer.


  1. ^ Bruch, Marcel; Monperrus, Martin; Mezini, Mira (2009). "Learning from examples to improve code completion systems". Proceedings of the 7th joint meeting of the European software engineering conference and the ACM SIGSOFT symposium on the foundations of software engineering on European software engineering conference and foundations of software engineering symposium - ESEC/FSE '09. p. 213. doi:10.1145/1595696.1595728. ISBN 9781605580012.
  2. ^ Autocomplete | Define Autocomplete at Retrieved on 2014-04-04.
  3. ^ FAQ - CodeBlocks. (2014-02-01). Retrieved on 2014-04-04.
  4. ^ Qt Documentation - Completing Code. Retrieved on 2015-07-07.
  5. ^ Using Dynamic Apex to retrieve Picklist Values | Developer Force Blog. (2008-12-09). Retrieved on 2014-04-04.
  6. ^ Murach. C# 2005. p. 56.
  7. ^ Home Page of Gail Kaiser. Retrieved on 2014-04-04.
  8. ^ Gail E. Kaiser at DBLP Bibliography Server
  9. ^ Earnest, Les. "The First Three Spelling Checkers" (PDF). Stanford University. Archived from the original (PDF) on 22 October 2012. Retrieved 10 October 2011.
  10. ^ Peterson, James (December 1980). Computer Programs for Detecting and Correcting Spelling Errors (PDF). Retrieved 18 February 2011.
  11. ^ Earnest, Les. Visible Legacies for Y3K (PDF). Archived from the original (PDF) on 20 July 2011. Retrieved 18 February 2011.
  12. ^ Using IntelliSense. Retrieved on 2014-04-04.
  13. ^ Visual Studio IntelliCode
  14. ^
  15. ^
  16. ^ Eclipse Code Recommenders: It’s all about intelligent code completion. (2010-05-03). Retrieved on 2014-04-04.
  17. ^ 542689 - Don't include Code Recommenders for 2018-12
  18. ^ cross-project-issues-dev Withdrawing Code Recommenders from SimRel
  19. ^ Archived Projects | The Eclipse Foundation
  20. ^ Vim Intellisense. Retrieved on 2014-04-04.

External links[edit]