dotnet/machinelearning · error · ArgumentException

PositiveNumberOfCharacters

Error message

PositiveNumberOfCharacters

What it means

TextFieldParser.PeekChars throws ArgumentException("PositiveNumberOfCharacters") when numberOfChars is zero or negative. PeekChars only makes sense for a positive count of characters to inspect in the next line, so non-positive arguments are rejected up front.

Source

Thrown at src/Microsoft.Data.Analysis/TextFieldParser.cs:453

                    return ParseFixedWidthLine();
                case FieldType.Delimited:
                    return ParseDelimitedLine();
                default:
                    Debug.Fail("The TextFieldType is not supported");
                    return null;
            }
        }

        ///<summary>
        /// Peek at <paramref name="numberOfChars"/> characters of the next data line without reading the line
        ///</summary>
        ///<param name="numberOfChars">The number of characters to look at in the next data line.</param>
        ///<returns>A string consisting of the first <paramref name="numberOfChars"/> characters of the next line. >If numberOfChars is greater than the next line, only the next line is returned</returns>
        public string PeekChars(int numberOfChars)
        {
            if (numberOfChars <= 0)
            {
                throw new ArgumentException(string.Format(Strings.PositiveNumberOfCharacters, nameof(numberOfChars)));
            }

            if ((_reader == null) || (_buffer == null))
            {
                return null;
            }

            if (_endOfData)
            {
                return null;
            }

            string line = PeekNextDataLine();
            if (line == null)
            {
                _endOfData = true;
                return null;
            }

View on GitHub (pinned to 7b76e69cf9)

Solutions

1. Pass a strictly positive integer to PeekChars (numberOfChars >= 1)
2. Guard the call site: only call PeekChars when the computed count is > 0
3. Fix the source of the non-positive value (config default, counter, calculation)

Example fix

// before
parser.PeekChars(previewLength); // previewLength could be 0
// after
string peeked = previewLength > 0 ? parser.PeekChars(previewLength) : null;

if (numberOfChars <= 0) throw new ArgumentOutOfRangeException(nameof(numberOfChars), "PeekChars requires a positive character count.");

try { peeked = parser.PeekChars(n); } catch (ArgumentException ex) when (ex.Message.Contains("PositiveNumberOfCharacters")) { peeked = null; }

When it happens

Trigger: Calling PeekChars(0) or PeekChars with a negative int, often when the value comes from a computed variable or user configuration rather than a literal.

Common situations: A config value for 'preview length' defaulting to 0; an off-by-one in decrementing a counter before calling PeekChars; passing an uninitialized/zero int field.

Related errors

• StreamDoesntSupportReading
• Specified argument was out of the range of valid values. (Pa
• index
• Strings.IndexIsGreaterThanColumnLength
• string.Format(Strings.MismatchedValueType, typeof(string))

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/7f8662009e55dd3b. Report an issue: GitHub.