How to Retrieve Dictionary Keys from Values in C#?

Retrieving Dictionary Keys by Value in C#

In C#, obtaining a dictionary key by its associated value requires an additional lookup operation beyond standard dictionary functionality. Here's how it can be achieved:

Using the FirstOrDefault() Method:

If values in the dictionary are not guaranteed to be unique, you can use the FirstOrDefault() method of the Enumerable class to find the first matching value:

// Dictionary with string keys and values
Dictionary<string, string> types = new Dictionary<string, string>()
{
    { "1", "one" },
    { "2", "two" },
    { "3", "three" }
};

// Get the key associated with the value "one"
string myKey = types.FirstOrDefault(x => x.Value == "one").Key;

In this case, myKey will contain the value "1". Note that this approach may not return a key if multiple values in the dictionary have the same value.

Creating an Inverse Dictionary:

Alternatively, if values are unique and are inserted less frequently than read, you can create an inverse dictionary, where values are keys and keys are values:

// Create an inverse dictionary
Dictionary<string, string> inverseTypes = new Dictionary<string, string>();

// Populate the inverse dictionary
foreach (var kvp in types)
{
    inverseTypes[kvp.Value] = kvp.Key;
}

// Get the key associated with the value "one"
string myKey = inverseTypes["one"];

With this approach, the lookup operation can be performed directly using the inverse dictionary without the need for a separate lookup. Keep in mind that this option involves additional memory overhead due to the creation of the inverse dictionary.

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