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Another two C# idioms - Part 3

canro91 profile image Cesar Aguirre Originally published at canro91.github.io Updated on ・2 min read

This part of the C# idioms series is only about dictionaries.

Instead of checking if a dictionary contains an item before adding it, use TryAdd

TryAdd will return if an item was added or not to the dictionary. Unlike Add, if the given key is already in the dictionary, TryAdd won't throw any exception. It will simply do nothing. The item is already there.

Before,

var myDictionary = new Dictionary<string, string>();
myDictionary.Add("foo", "bar");

// System.ArgumentException: An item with the same key has already been added. Key: foo
myDictionary.Add("foo", "baz");
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After,

var myDictionary = new Dictionary<string, string>();

if (!myDictionary.ContainsKey("foo"))
  myDictionary.Add("foo", "bar");
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Even better,

var myDictionary = new Dictionary<string, string>();
myDictionary.TryAdd("foo", "bar"); // true

myDictionary.Add("foo", "baz");
myDictionary.TryAdd("foo", "bar"); // false
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Avoid KeyNotFoundException with TryGetValue or GetValueOrDefault

At least now, the KeyNotFoundException message contains the name of the not-found key. The old days chasing the not-found key are over.

On one hand, TryGetValue uses an output parameter with the found value. It outputs a default value when the dictionary doesn't contain the item. TryGetValue dates back to the days without tuples.

On another hand, GetValueOrDefault returns a default value or one you provide if the key wasn't found.

Before,

var dict = new Dictionary<string, string>();

// System.Collections.Generic.KeyNotFoundException: The given key 'foo' was not present in the dictionary.
dict["foo"];
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After,

var dict = new Dictionary<string, string>();

dict.TryGetValue("foo", out var foo); // false, foo -> null

dict.Add("foo", "bar");
dict.TryGetValue("foo", out foo); // true, foo -> "bar"
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Even better,

var dict = new Dictionary<string, string>();

dict.GetValueOrDefault("foo"); // null
dict.GetValueOrDefault("foo", "withoutFoo"); // "withoutFoo"

dict.Add("foo", "bar");
dict.GetValueOrDefault("foo", "withoutFoo"); // "bar"
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Happy coding!

Discussion (1)

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wellswang profile image
Wells Wang

haha, great