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C# KeyNotFoundException: Key Not Present in DictionaryUnderstand KeyNotFoundException and why it is encountered when using a Dictionary.
KeyNotFoundException. In a C# program, a KeyNotFoundException was thrown. This is likely caused by a lookup done on a key (one that is not present) in a Dictionary collection.
A solution. As always we want a quick way to fix the problem. We can use an if-statement with TryGetValue to avoid this exception.
ContainsKey
TryGetValue
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.
Here The KeyNotFoundException is thrown on the final line of the try-block. The string "test" is not present in the collection.
Try
Catch
C# program that throws KeyNotFoundException
using System; using System.Collections.Generic; class Program { static void Main() { try { // // Create new Dictionary with string key of "one" // Dictionary<string, string> test = new Dictionary<string, string>(); test.Add("one", "value"); // // Try to access key of "two" // string value = test["two"]; } catch (Exception ex) { // // 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.
ContainsKey
TryGetValue
Important We use if-statement when testing values in the Dictionary, because there is always a possibility that the key will not exist.
If
Runtime The C# compiler cannot detect missing keys. They can only be detected at runtime.
Dictionary
C# program that does not throw
using System; using System.Collections.Generic; class Program { static void Main() { Dictionary<string, string> 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
Summary. We saw how to raise and catch the KeyNotFoundException during runtime. We then saw how to avoid causing the exception. We looked at a program that does not have this problem.
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