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Posted 1 Feb 2013


, 27 Mar 2015
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Extension methods to select a Dictionary entry using StartsWith


I present here two extension methods that allow me to select an item from a Dictionary even if I have only the first part of the key, rather than the whole key.


A few times in the past year, I wanted to select an entry in a Dictionary<string,T> based on only the first part of the key. In early attempts, I added each portion separately; for example if I had a key of "YES", I would also add "Y" and "YE". This became unwieldy as the keys became longer and the benefits of hashing didn't seem as crucial. However, I didn't want to depart from using a Dictionary entirely.

Adding the use of A String.StartsWith that uses a StringComparer[^] to a Dictionary was simple.

The Code

BestMatch will attempt the usual ContainsKey first and only resort to iterating the keys and using StartsWith if the full string isn't found. If it does have to iterate, then it can't stop as soon as it finds a match, it must continue in case it finds a second match.

public static T
  this System.Collections.Generic.Dictionary<string,T> Source
  string                                               String
  T result = default(T) ;

  if ( Source.ContainsKey ( String ) )
    result = Source [ String ] ;
    bool found = false ;

    foreach ( string s in Source.Keys )
      if ( s.StartsWith ( String , Source.Comparer ) )
        if ( found )
          throw ( new System.ArgumentException ( "Ambiguous name" , String ) ) ;

        result = Source [ s ] ;

        found = true ;

    if ( !found )
      throw ( new System.ArgumentException ( "Unrecognized name" , String ) ) ;

  return ( result ) ;

The second method works similarly to TryParse in that it returns a boolean and the value (or the default for the type) is passed back via an out parameter. If you want to change the name to TryBestMatch or HasBestMatch or something, go ahead.

public static bool
  this System.Collections.Generic.Dictionary<string,T> Source
  string                                               String
  out T                                                Value
  bool result = true ;

  Value = default(T) ;

    Value = BestMatch ( Source , String ) ;
    result = false ;

  return ( result ) ;


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Software Developer (Senior)
United States United States
BSCS 1992 Wentworth Institute of Technology

Originally from the Boston (MA) area. Lived in SoCal for a while. Now in the Phoenix (AZ) area.

OpenVMS enthusiast, ISO 8601 evangelist, photographer, opinionated SOB, acknowledged pedant and contrarian


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Comments and Discussions

GeneralMy vote of 3 Pin
Paulo Zemek5-Feb-13 5:23
memberPaulo Zemek5-Feb-13 5:23 
GeneralRe: My vote of 3 Pin
PIEBALDconsult5-Feb-13 6:08
memberPIEBALDconsult5-Feb-13 6:08 
GeneralRe: My vote of 3 Pin
Paulo Zemek5-Feb-13 6:29
memberPaulo Zemek5-Feb-13 6:29 
GeneralRe: My vote of 3 Pin
PIEBALDconsult5-Feb-13 6:35
memberPIEBALDconsult5-Feb-13 6:35 
GeneralRe: My vote of 3 Pin
Paulo Zemek5-Feb-13 6:41
memberPaulo Zemek5-Feb-13 6:41 
GeneralRe: My vote of 3 Pin
PIEBALDconsult5-Feb-13 7:17
memberPIEBALDconsult5-Feb-13 7:17 
GeneralRe: My vote of 3 Pin
Paulo Zemek5-Feb-13 7:49
memberPaulo Zemek5-Feb-13 7:49 
GeneralRe: My vote of 3 Pin
PIEBALDconsult6-Feb-13 19:05
memberPIEBALDconsult6-Feb-13 19:05 
QuestionAlternative version? Pin
George Swan2-Feb-13 23:47
memberGeorge Swan2-Feb-13 23:47 
SuggestionRe: Alternative version? Pin
Edo Tzumer3-Feb-13 0:36
memberEdo Tzumer3-Feb-13 0:36 
GeneralRe: Alternative version? Pin
George Swan3-Feb-13 3:23
memberGeorge Swan3-Feb-13 3:23 

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