dotnet/machinelearning · error · InvalidOperationException

NullFieldWidths

Error message

NullFieldWidths

What it means

TextFieldParser.ValidateFieldWidths throws InvalidOperationException("NullFieldWidths") when ReadFields is called in FixedWidth mode but the _fieldWidths array was never set. Fixed-width parsing requires the field widths to be configured beforehand via SetFieldWidths or the FieldWidths property.

Source

Thrown at src/Microsoft.Data.Analysis/TextFieldParser.cs:947

            }
        }

        private void ValidateFixedWidthLine(StringInfo line, long lineNumber)
        {
            Debug.Assert(line != null, "No Line sent");
            if (line.LengthInTextElements < _lineLength)
            {
                _errorLine = line.String;
                _errorLineNumber = checked(_lineNumber - 1);
                throw new Exception(string.Format(Strings.CannotParseWithFieldWidths, lineNumber));
            }
        }

        private void ValidateFieldWidths()
        {
            if (_fieldWidths == null)
            {
                throw new InvalidOperationException(Strings.NullFieldWidths);
            }
            if (_fieldWidths.Length == 0)
            {
                throw new InvalidOperationException(Strings.EmptyFieldWidths);
            }
            checked
            {
                int widthBound = _fieldWidths.Length - 1;
                _lineLength = 0;
                int num = widthBound - 1;
                for (int i = 0; i <= num; i++)
                {
                    Debug.Assert(_fieldWidths[i] > 0, "Bad field width, this should have been caught on input");
                    _lineLength += _fieldWidths[i];
                }
                if (_fieldWidths[widthBound] > 0)
                {
                    _lineLength += _fieldWidths[widthBound];

View on GitHub (pinned to 7b76e69cf9)

Solutions

1. Call parser.SetFieldWidths(new[] { 5, 10, 15 }) before ReadFields when using FixedWidth
2. Or set TextFieldType = FieldType.Delimited and provide Delimiters instead if your data is actually delimited
3. Verify the code path that assigns FieldWidths actually executes before reading

Example fix

// before
parser.TextFieldType = FieldType.FixedWidth;
var fields = parser.ReadFields(); // InvalidOperationException: NullFieldWidths
// after
parser.TextFieldType = FieldType.FixedWidth;
parser.SetFieldWidths(new[] { 10, 20, 15 });
var fields = parser.ReadFields();

if (parser.TextFieldType == FieldType.FixedWidth && (parser.FieldWidths == null || parser.FieldWidths.Count == 0)) throw new InvalidOperationException("FieldWidths must be set before ReadFields in FixedWidth mode");

try { fields = parser.ReadFields(); } catch (InvalidOperationException ex) when (ex.Message.Contains("NullFieldWidths") || ex.Message.Contains("EmptyFieldWidths")) { /* configure widths then retry */ }

When it happens

Trigger: Setting TextFieldType = FieldType.FixedWidth but never calling SetFieldWidths / setting FieldWidths, then calling ReadFields. Also triggered by passing a null array to the setter paths that route into ValidateFieldWidths.

Common situations: Copy-pasting a delimited-file parser setup and switching TextFieldType to FixedWidth without adding widths; a code path that conditionally sets widths but the condition didn't fire; widths cleared by resetting the parser config.

Understand the failure class

Background: "is required", "must be set", "missing required field": configuration validation errors across open-source libraries — this error's family across 36 libraries.

Related errors

• PositiveNumberOfCharacters
• StreamDoesntSupportReading
• FileNotFound
• EmptyFieldWidths
• You need to configure a default, not named, model before you

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/c87670e6c3fb6447. Report an issue: GitHub.