jstedfast/MailKit · error · ArgumentNullException

outputBuffer

Error message

outputBuffer

What it means

DES.TransformBlock throws ArgumentNullException with parameter name "outputBuffer" when the destination array is null. After validating the input, it checks that a real buffer exists to receive the encrypted/decrypted block before writing to outputOffset.

Solutions

1. Allocate an output array of at least 8 bytes at outputOffset before calling.
2. If you intended in-place transformation, pass the input buffer as the output buffer (DES allows same-buffer operation at the same offset).
3. Verify the output variable assignment isn't conditional code that skipped initialization.

Example fix

// before
des.TransformBlock(input, 0, 8, output, 0); // output null

// after
var output = new byte[8];
des.TransformBlock(input, 0, 8, output, 0);

var output = outputBuffer ?? new byte[8];
des.TransformBlock(input, 0, 8, output, 0);

static bool CanHoldBlock(byte[] buf, int off) => buf != null && off + 8 <= buf.Length;

try {
    des.TransformBlock(input, 0, 8, output, 0);
} catch (ArgumentNullException ex) when (ex.ParamName == "outputBuffer") {
    // allocate output and retry
    output = new byte[8];
}

When it happens

Trigger: Calling des.TransformBlock(input, 0, 8, null, 0) — outputBuffer null even when you intend only to measure size (not supported by this transform).

Common situations: APIs that allow a null output to query required size being misused against this block transform; an uninitialized output array variable; refactoring that changed in-place transformation to separate output buffers.

Related errors

• inputBuffer
• inputOffset
• inputCount
• Can only transform 8 bytes at a time.
• outputOffset

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/ea345cfb96f4d0c2. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/Security/Ntlm/DES.cs:120

				get { return 8; }
			}

			public int TransformBlock (byte[] inputBuffer, int inputOffset, int inputCount, byte[] outputBuffer, int outputOffset)
			{
				if (inputBuffer == null)
					throw new ArgumentNullException ("inputBuffer");

				if (inputOffset < 0 || inputOffset > inputBuffer.Length)
					throw new ArgumentOutOfRangeException ("inputOffset");

				if (inputCount < 0 || inputOffset > inputBuffer.Length - inputCount)
					throw new ArgumentOutOfRangeException ("inputCount");

				if (inputCount != 8)
					throw new ArgumentOutOfRangeException ("inputCount", "Can only transform 8 bytes at a time.");

				if (outputBuffer == null)
					throw new ArgumentNullException ("outputBuffer");

				if (outputOffset < 0 || outputOffset > outputBuffer.Length - 8)
					throw new ArgumentOutOfRangeException ("outputOffset");

				return engine.ProcessBlock (inputBuffer, inputOffset, outputBuffer, outputOffset);
			}

			public byte[] TransformFinalBlock (byte[] inputBuffer, int inputOffset, int inputCount)
			{
				if (inputBuffer == null)
					throw new ArgumentNullException ("inputBuffer");

				if (inputOffset < 0 || inputOffset > inputBuffer.Length)
					throw new ArgumentOutOfRangeException ("inputOffset");

				if (inputCount < 0 || inputOffset > inputBuffer.Length - inputCount)
					throw new ArgumentOutOfRangeException ("inputCount");

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