You want to improve your Dictionary usage by avoiding a common mistake. One example is incrementing the value of a Dictionary at a key. Code that uses ContainsKey for this is doing more than you realize, and the TryGetValue method offers an excellent solution. Here we see examples of using the TryGetValue method in the C# programming language.
Avoid this common Dictionary mistake.
Skip duplicate hash code computations and lookups.
Using TryGetValue method
First, we look at how you can rewrite programs that use ContainsKey with TryGetValue. Even developers who understand Dictionary and how hashtable lookups work can make this mistake. It also gives us an interesting insight into how hashtables in C# work. We apply the TryGetValue method in the C# language.
=== Code that uses ContainsKey (C#) ===using System.Collections.Generic;class Program{ static void Main() { var d = new Dictionary<string, int>(); d.Add("key", 0); // // A. // Does three lookups in Dictionary. // if (d.ContainsKey("key")) { d["key"] ; } }}=== Code that shows what ContainsKey does (C#) ===using System.Collections.Generic;class Program{ static void Main() { var d = new Dictionary<string, int>(); d.Add("key", 0); // // B. // This code is equivalent to A. // if (d.ContainsKey("key")) { d["key"] = d["key"] 1; } }}=== Code that uses TryGetValue (C#) ===using System.Collections.Generic;class Program{ static void Main() { var d = new Dictionary<string, int>(); d.Add("key", 0); // // C. // This code does two hash lookups. // int value; if (d.TryGetValue("key", out value)) { d["key"] = value 1; } }}
Description of first example. The problematic code increments or decrements the value of a Dictionary at a key. Skilled developers usually increment using the operator. However, on a Dictionary this is a bad choice.
Second example. The increment does two lookups when you really only need one in the common case above. To increment the value of the Dictionary, you have to get the current value.
See Dictionary Lookup Comparison.
Third example with TryGetValue. Every lookup in a hash on a string key has to compute the hash code, which has a performance penalty. To solve this inefficiency, use the TryGetValue method. You can store the value it finds.
Benchmark
Here we look at how the TryGetValue method performs when used in the way in the above example. I am so used to using the operator (or --) on integers, that this slipped through my review. Expert developers understand precisely what their code is doing. So knowing this is much more important than simply making the code twice as fast.
=== Benchmark results for TryGetValue (C#) ===Use ContainsKey: 1700 msUse TryGetValue: 1108 ms [faster]
Summary
Here we looked at how you can rewrite your Dictionary code to use TryGetValue instead of ContainsKey. Don't use the or -- operators on Dictionary. Instead, store the value with TryGetValue. Careful and precise understanding of how hashtable lookups work in .NET and C# helps improve your code significantly.
