How to Find Item in Dictionary Collection?

Question

I have declared and populated the following collection.

protected static Dictionary<string, string> _tags; 

Now I want to look locate a particular entry in the collection. I tried the following.

thisTag = _tags.FirstOrDefault(t => t.Key == tag); if (thisTag != default(KeyValuePair<string, string>))     ... 

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.)

Answer 1

thisTag = _tags.FirstOrDefault(t => 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 */ } 

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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