How to Find Item in Dictionary Collection

Question

I have declared and populated the following collection   protected static Dictionary lt string  string gt   tags    Now I want to look locate a particular entry in the collection  I tried the following   thisTag    tags FirstOrDefault t   gt  t Key    tag   if  thisTag    default KeyValuePair lt string  string gt              And I get the error      Operator      cannot be applied to operands of type  System Collections Generic KeyValuePair  and      Initially I attempted to compare the result to null  and I guess that s not supported with structs   I would ve thought that finding an item within a collection is a very trivial task  So how the heck to I determine if the item I m looking for was found    Note  I m using Dictionary because I want fast lookups  I know I can use Contains   to determine if the item is there  But that means a total of two lookups  which sort of defeats the purpose of having a fast lookup  I ll happily using a different collection if it can quickly lookup an item and I have a way to determine if it was successful

User · Answer

Sometimes you still need to use FirstOrDefault if you have to do different tests. If the Key component of your dictionnary is nullable, you can do this:

thisTag = _tags.FirstOrDefault(t => t.Key.SubString(1,1) == 'a');
if(thisTag.Key != null) { ... }

Using FirstOrDefault, the returned KeyValuePair's key and value will both be null if no match is found.

User · Answer

It s possible to find the element in Dictionary collection by using ContainsKey or TryGetValue as follows   class Program       protected static Dictionary lt string  string gt   tags   new Dictionary lt string string gt          static void Main string   args                string strValue            tags Add  101    C              tags Add  102    ASP NET             if   tags ContainsKey  101                          strValue    tags  101                Console WriteLine strValue                      if   tags TryGetValue  101   out strValue                         Console WriteLine strValue

User · Answer

Of course  if you want to make sure it s in there otherwise fail then this works   thisTag    tags key    NOTE  This will fail if the key value pair does not exists but sometimes that is exactly what you want  This way you can catch it and do something about the error  I would only do this if I am certain that the key value pair is or should be in the dictionary and if not I want it to know about it via the throw

User · Answer

thisTag    tags FirstOrDefault t   gt  t Key    tag     is an inefficient and a little bit strange way to find something by key in a dictionary  Looking things up for a Key is the basic function of a Dictionary   The basic solution would be   if   tags Containskey tag     string myValue    tags tag           But that requires 2 lookups    TryGetValue key  out value  is more concise and efficient  it only does 1 lookup  And that answers the last part of your question  the best way to do a lookup is   string myValue  if   tags TryGetValue tag  out myValue        use myValue          VS 2017 update  for C  7 and beyond we can declare the result variable inline   if   tags TryGetValue tag  out string myValue            use myValue       use myValue  still in scope  null if not found

[c#] How to Find Item in Dictionary Collection?

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