TheAlgorithms/C-Sharp · error · ArgumentNullException

Input cannot be null

Error message

Input cannot be null

What it means

GetPaddingCount rejects input that is null (or lacks valid PKCS7 padding) and reports it as a generic ArgumentException instead of ArgumentNullException, since the method requires a non-null padded buffer to count trailing padding bytes.

Solutions

1. Check the array for null before calling GetPaddingCount.
2. Ensure the decrypt step always returns a buffer (or throw) instead of null.
3. If null means 'no data', skip unpadding entirely rather than calling.
4. Fail fast with a clear message at the data-source boundary.

Example fix

// before
var count = padding.GetPaddingCount(decrypted); // decrypted may be null
// after
if (decrypted is null) throw new InvalidOperationException("Decryption produced no data");
var count = padding.GetPaddingCount(decrypted);

if (decrypted is null) throw new InvalidOperationException("Decryption returned null");

static bool IsUnpaddable(byte[]? input) => input is not null && input.Length > 0;

try { count = padding.GetPaddingCount(input); }
catch (ArgumentNullException) { count = -1; /* treat as no data */ }

When it happens

Trigger: Calling GetPaddingCount(null) — typically when a decryption step returned null (failed decrypt API, null from a cache or deserializer) and the result was passed straight through.

Common situations: A decrypt method that returns null on failure; nullable byte[] from external data pipelines; forgetting to handle an empty/failed read before unpadding.

Related errors

• Invalid block size
• Not enough space in input array for padding
• Input length must be a multiple of block size
• Invalid padding length
• Invalid padding

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/dec1317364544250. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Crypto/Paddings/Pkcs7Padding.cs:132

        return output;
    }

    /// <summary>
    /// Gets the number of padding bytes in the given input data according to the PKCS7 padding scheme.
    /// </summary>
    /// <param name="input">The input data with PKCS7 padding. Must not be null and must have a valid padding.</param>
    /// <returns>The number of padding bytes in the input data.</returns>
    /// <exception cref="ArgumentException">
    /// Thrown if the input data is null or has an invalid padding.
    /// </exception>
    /// <remarks>
    /// This method uses bitwise operations to avoid branching.
    /// </remarks>
    public int GetPaddingCount(byte[] input)
    {
        if (input == null)
        {
            throw new ArgumentNullException(nameof(input), "Input cannot be null");
        }

        // Get the last byte of the input data as the padding value.
        var lastByte = input[^1];
        var paddingCount = lastByte & 0xFF;

        // Calculate the index where the padding starts
        var paddingStartIndex = input.Length - paddingCount;
        var paddingCheckFailed = 0;

        // Check if the padding start index is negative or greater than the input length.
        // This is done by using bitwise operations to avoid branching.
        // If the padding start index is negative, then its most significant bit will be 1.
        // If the padding count is greater than the block size, then its most significant bit will be 1.
        // By ORing these two cases, we can get a non-zero value rif either of them is true.
        // By shifting this value right by 31 bits, we can get either 0 or -1 as the result.
        paddingCheckFailed = (paddingStartIndex | (paddingCount - 1)) >> 31;

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