KeyNotFoundException. When using a Dictionary, we often access many different keys—and sometimes a key may not exist. In this case, a KeyNotFoundException may be thrown.

This exception means we tried to access a key that does not exist, in a way that does not prevent exceptions. To fix this problem in VB.NET, we can use a function like TryGetValue.

Example. Here we place a Dictionary key access inside a Try block, and then in the Catch block, we print the exception. This helps us understand more about the error.

Module Module1
    Sub Main()

        Try

            ' Part 1: create a Dictionary and add 1 key.
            Dim test = New Dictionary(Of String, String)()
            test.Add("one", "value")

            ' Part 2: access a key that does not exist.
            Dim value = test("two")

        Catch ex as Exception

            ' Part 3: print the exception.
            Console.WriteLine(ex)

        End Try

    End Sub
End Module
System.Collections.Generic.KeyNotFoundException:
The given key 'two' was not present in the dictionary.
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at ...

Example 2. How can we fix a KeyNotFoundException? We can use TryGetValue instead of the indexer to access a key. Then we place an If-Else around the TryGetValue call.

Module Module1
    Sub Main()

        Dim test = New Dictionary(Of String, String)()
        test.Add("one", "value")

        ' Use TryGetValue for exception-free code.
        Dim value As String = Nothing
        If test.TryGetValue("two", value)
            Console.WriteLine("Found")
        Else
            Console.WriteLine("Not found")
        End If

    End Sub
End Module
Not found

Summary. Using a function like TryGetValue (or GetValueOrDefault) is the best way to avoid the KeyNotFoundException. Usually an If-Else block must be placed around the new function call.