Aim

To accept a character from the keyboard and decide whether it is a vowel, demonstrating case normalisation with char.ToLower and short-circuit boolean composition.

Theory

A vowel test is a set-membership problem. The professional solution normalises case FIRST (char.ToLower) so a single comparison chain covers both 'E' and 'e' — halving the number of conditions and removing a whole class of copy-paste bugs. The chain ch == 'a' || ch == 'e' || ... uses the short-circuit OR operator: evaluation stops at the first operand that is true, so on average fewer than three comparisons execute. Character literals in C# are genuine char values compared by their UTF-16 code units — an O(1) integer comparison with no string machinery involved. Alternatives worth knowing for the viva: "aeiou".Contains(ch) (readable but a linear scan through a string), a C# 9 pattern ch is 'a' or 'e' or 'i' or 'o' or 'u' (compiled to an efficient compare/jump sequence), and HashSet<char> for large alphabets. Finally, char.ToLower(ch) is culture-sensitive; char.ToLowerInvariant avoids the famous Turkish-I problem ('I' lowercases to dotless 'ı' under the tr-TR culture) — a real internationalisation bug class in shipped software.

Requirements

• .NET SDK 8.0+ (dotnet new console, dotnet run) or the RUN CODE button.

Procedure

1. Create the project: dotnet new console -n VowelCheck, then cd VowelCheck.
2. Replace Program.cs with the given code.
3. Execute dotnet run and type a character such as E.
4. Test the boundary cases: uppercase vowel, consonant, digit, and empty input (defaults to 'e').

Explanation of the Code

• char.ToLower(string.IsNullOrWhiteSpace(input) ? 'e' : input[0]) — one expression performs three jobs: null guard, default selection, and case normalisation. After this line only lowercase letters need testing.
• bool isVowel = ch == 'a' || ch == 'e' || ch == 'i' || ch == 'o' || ch == 'u'; — the result of the boolean expression is stored directly into a bool; no if statement is needed to "manufacture" a boolean value.
• Console.WriteLine(isVowel ? $"{ch} is a vowel." : $"{ch} is not a vowel."); — the ternary selects between two interpolated strings; only the chosen string is ever constructed at run time.

Expected Output

Interactive session:

Enter a character (default e): K
k is not a vowel.

Note the echo is lowercase because normalisation happened before printing. With empty input the default path prints e is a vowel.; with O it prints o is a vowel.

🎯 Viva Questions

1. Why normalise case before comparing? — One comparison set handles both cases; fewer branches, fewer bugs.
2. What is short-circuit evaluation? — || stops at the first true operand; && stops at the first false.
3. Difference between char.ToLower and char.ToLowerInvariant? — The first honours the current culture (Turkish-I pitfall); the second uses invariant rules and is deterministic.
4. How would a C# 9 pattern express this test? — ch is 'a' or 'e' or 'i' or 'o' or 'u'.
5. Is 'a' a string? — No; single quotes create a char literal (one UTF-16 code unit), double quotes create a string.
6. Complexity of the comparison chain? — O(1): at most five integer comparisons, fewer on average due to short-circuiting.