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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.)
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In C#, Dictionary is a generic collection which is generally used to store key/value pairs. The working of Dictionary is quite similar to the non-generic hashtable. The advantage of Dictionary is, it is generic type. Dictionary is defined under System.
noun. the act of collecting. something that is collected; a group of objects or an amount of material accumulated in one location, especially for some purpose or as a result of some process: a stamp collection;a collection of unclaimed hats in the checkroom;a collection of books on Churchill.
Syntax: public bool ContainsKey (TKey key); Here, the key is the Key which is to be located in the Dictionary. Return Value: This method will return true if the Dictionary contains an element with the specified key otherwise, it returns false.
Performance Considerations While Using Index So using The Dictionary<TKey,TValue> as an array has a performance penalty. The Dictionary<TKey,TValue> has no way to access directly the nth “element” via ElementAt. If we want to access the nth “element”, the Dictionary needs to enumerate all of the entries in it.
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 */ } 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 If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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