KeyNotFoundException

In a C# program, a KeyNotFoundException can be thrown. This is likely caused by a lookup done on a key (one that is not present) in a Dictionary collection.

As always we want a quick way to fix the problem. We can use an if-statement with TryGetValue to avoid this exception.

Example

Here we see some code that looks correct. But it has a severe flaw. You cannot look up a key that is not found in the Dictionary and try to assign your variable to its value.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        try
        {
            // Part 1: create new Dictionary with string key "one".
            var test = new Dictionary<string, string>();
            test.Add("one", "value");

            // Part 2: try to access key "two".
            string value = test["two"];
        }
        catch (Exception ex)
        {
            // Part 3: an exception will be thrown.
            Console.WriteLine(ex);
        }
    }
}
System.Collections.Generic.KeyNotFoundException:
    The given key was not present in the dictionary.
at System.ThrowHelper.ThrowKeyNotFoundException()
...

Example 2

We can fix this exception by using the TryGetValue method. Note that could use ContainsKey instead of TryGetValue. But we preserve the intention of the previous code here.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        var test = new Dictionary<string, string>();
        test.Add("one", "value");

        // Use TryGetValue to avoid KeyNotFoundException.
        string value;
        if (test.TryGetValue("two", out value))
        {
            Console.WriteLine("Found");
        }
        else
        {
            Console.WriteLine("Not found");
        }
    }
}
Not found

It is possible to raise and catch the KeyNotFoundException during execution. But it is best to avoid causing the exception—we tested a program that does not have this problem.