TryGetValue

This method optimizes Dictionary usage. It gets a value (at a key) from a Dictionary. And it eliminates unneeded lookups, making programs better.

With TryGetValue, we can combine the "try" part of seeing if a key exists, and the "get" part of getting the existing value. This saves 1 lookup.

TryGetValue, example

We can rewrite programs that use ContainsKey with TryGetValue. By combining 2 operations (try and get) into 1, we can optimize and simplify programs.

using System;
using System.Collections.Generic;

var counts = new Dictionary<string, int>();
counts.Add("key", 0);
// Keep the result of TryGetValue.
int value;
if (counts.TryGetValue("key", out value))
{
    counts["key"] = value + 1;
    Console.WriteLine("VALUE: " + counts["key"]);
}
VALUE: 1

Out type

The second argument to TryGetValue is an out parameter. We can specify the type (here it is bool) directly in the call to the TryGetValue method.

using System;
using System.Collections.Generic;

var ids = new Dictionary<string, bool>() { { "X1", true } };
// We can specify the "out" type by argument.
if (ids.TryGetValue("X1", out bool result))
{
    Console.WriteLine($"VALUE: {result}");
}
VALUE: True

Benchmark, TryGetValue

This benchmark compares the TryGetValue method against ContainsKey and indexer. It compares 1 lookup against 2 lookups.

using System;
using System.Collections.Generic;
using System.Diagnostics;

const int _max = 10000000;
var test = new Dictionary<string, int>();
test["key"] = 1;
int sum = 0;

// Version 1: use ContainsKey and access the key again for its value.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    if (test.ContainsKey("key"))
    {
        sum += test["key"];
    }
}
s1.Stop();

// Version 2: use TryGetValue and use the key already accessed.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    if (test.TryGetValue("key", out int result))
    {
        sum += result;
    }
}
s2.Stop();
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
38.11 ns    ContainsKey, indexer
21.16 ns    TryGetValue

Summary

We can rewrite Dictionary code to use TryGetValue instead of ContainsKey. It is good to avoid the increment or decrement operators here. Instead store the value with TryGetValue.