Case, Dictionary. Consider a VB.NET Dictionary that has String keys—the key "Cat" is different than the key "cat." This makes sense, as the characters are different.

But sometimes we want case-insensitive lookups, where uppercase and lowercase are the same. We can specify StringComparer OrdinalIgnoreCase to achieve this effect.

Example. This VB.NET program creates a Dictionary specifies a custom IEqualityComparer in its constructor. It demonstrates the use of case-insensitivity in Dictionary keys.

IEqualityComparer

Dictionary GetValueOrDefault

Module Module1
    Sub Main()

        ' Step 1: create Dictionary specifying OrdinalIgnoreCase.
        Dim test = New Dictionary(Of String, Integer)(StringComparer.OrdinalIgnoreCase)

        ' Step 2: add some case-insensitive string keys.
        test.Add("Cat", 2)
        test.Add("Python", 4)
        test.Add("DOG", 6)

        ' Step 3: test one key with different cases.
        Dim result1 = test.GetValueOrDefault("cat")
        Dim result2 = test.GetValueOrDefault("CAT")
        Console.WriteLine($"   Cat: {result1} = {result2}")

        ' Step 4: test another key.
        Dim result3 = test.GetValueOrDefault("PYTHON")
        Dim result4 = test.GetValueOrDefault("Python")
        Console.WriteLine($"Python: {result3} = {result4}")

        ' Step 5: print out all entries in the Dictionary.
        For Each pair in test
            Console.WriteLine(pair)
        Next

    End Sub
End Module
   Cat: 2 = 2
Python: 4 = 4
[Cat, 2]
[Python, 4]
[DOG, 6]

IEqualityComparer. When we specify the StringComparer, we are changing the IEqualityComparer of the Dictionary. This will affect how many functions, like TryGetValue and ContainsKey, work.

Summary. Given that Windows uses case-insensitive file names, using a case-insensitive Dictionary is often helpful. Many VB.NET programs can benefit from StringComparer OrdinalIgnoreCase.