TheAlgorithms/C-Sharp · error · ArgumentOutOfRangeException

key must be non-empty string

Error message

key must be non-empty string

What it means

VigenereEncoder.AppendKey extends the key to the required cipher length by repetition, and rejects a null or empty key because repetition can never produce characters from nothing. An ArgumentOutOfRangeException with message "key must be non-empty string" is thrown when string.IsNullOrEmpty(key).

Solutions

1. Supply a non-empty alphabetic key when calling VigenereEncoder.Cipher.
2. Validate the key at the configuration/request boundary before invoking the encoder.
3. Provide a fallback default key when configured key is missing, if acceptable for your use case.
4. Catch ArgumentOutOfRangeException around Cipher and surface a clear 'encryption key required' message to the user.

Example fix

// before
var encoded = vigenere.Cipher(message, config.Key); // config.Key == ""
// after
if (string.IsNullOrEmpty(config.Key))
    throw new InvalidOperationException("Encryption key must be configured (Vigenere:Key).");
var encoded = vigenere.Cipher(message, config.Key);

if (string.IsNullOrWhiteSpace(key))
    throw new InvalidOperationException("A non-empty Vigenere key is required.");

static bool IsValidVigenereKey(string key) => !string.IsNullOrEmpty(key) && key.All(char.IsLetter);

try
{
    var encoded = vigenere.Cipher(message, key);
}
catch (ArgumentOutOfRangeException ex)
{
    throw new InvalidOperationException("Encryption key is missing or empty; configure Vigenere:Key.", ex);
}

When it happens

Trigger: Calling VigenereEncoder.Cipher (which calls AppendKey) with a null or "" key, e.g. Cipher("plaintext", "") or Cipher("plaintext", null) — throws ArgumentOutOfRangeException.

Common situations: Reading the key from configuration or an environment variable that is unset/empty; a UI or CLI passing an empty key field; deserializing a request where the key property defaulted to empty string.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

• No padding found.
• message
• Invalid block size
• Not enough space in input array for padding
• Input length must be a multiple of block size

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/88fbd051a2ad23f3. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Encoders/VigenereEncoder.cs:56

            if (!char.IsLetter(text[i]))
            {
                _ = encodedTextBuilder.Append(text[i]);
                continue;
            }

            var letterZ = char.IsUpper(key[i]) ? 'Z' : 'z';
            var encodedSymbol = symbolCipher(text[i].ToString(), letterZ - key[i]);
            _ = encodedTextBuilder.Append(encodedSymbol);
        }

        return encodedTextBuilder.ToString();
    }

    private string AppendKey(string key, int length)
    {
        if (string.IsNullOrEmpty(key))
        {
            throw new ArgumentOutOfRangeException($"{nameof(key)} must be non-empty string");
        }

        var keyBuilder = new StringBuilder(key, length);
        while (keyBuilder.Length < length)
        {
            _ = keyBuilder.Append(key);
        }

        return keyBuilder.ToString();
    }
}

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