dotnet/machinelearning · error · ArgumentException

Null column string was specified as {columnPurpose} in colum

Error message

Null column string was specified as {columnPurpose} in column information

What it means

Thrown by ValidateColumnInfoEnumerationProperty when one of the enumerable column-name properties of ColumnInformation (categorical, text, ignored, etc.) contains a null string element. The library forbids null entries because a null name cannot identify a column, and the message reports which purpose (columnPurpose) the offending list represents.

Source

Thrown at src/Microsoft.ML.AutoML/Utils/UserInputValidationUtil.cs:152

            allColumns.Add(columnInformation.LabelColumnName);
            if (columnInformation.ExampleWeightColumnName != null) { allColumns.Add(columnInformation.ExampleWeightColumnName); }
            if (columnInformation.CategoricalColumnNames != null) { allColumns.AddRange(columnInformation.CategoricalColumnNames); }
            if (columnInformation.NumericColumnNames != null) { allColumns.AddRange(columnInformation.NumericColumnNames); }
            if (columnInformation.TextColumnNames != null) { allColumns.AddRange(columnInformation.TextColumnNames); }
            if (columnInformation.IgnoredColumnNames != null) { allColumns.AddRange(columnInformation.IgnoredColumnNames); }

            var duplicateColName = FindFirstDuplicate(allColumns);
            if (duplicateColName != null)
            {
                throw new ArgumentException($"Duplicate column name {duplicateColName} is present in two or more distinct properties of provided column information", nameof(columnInformation));
            }
        }

        private static void ValidateColumnInfoEnumerationProperty(IEnumerable<string> columns, string columnPurpose)
        {
            if (columns?.Contains(null) == true)
            {
                throw new ArgumentException($"Null column string was specified as {columnPurpose} in column information");
            }
        }

        private static void ValidateLabelColumn(string labelColumn)
        {
            if (labelColumn == null)
            {
                throw new ArgumentException("Provided label column cannot be null");
            }
        }

        private static void ValidatePath(string path)
        {
            if (path == null)
            {
                throw new ArgumentNullException(nameof(path), "Provided path cannot be null");
            }

View on GitHub (pinned to 7b76e69cf9)

Solutions

Filter nulls from the enumeration before assigning: names.Where(n => n != null)
Fix the source (config parsing, array allocation) that produced null entries
Replace fixed-size array allocation with explicitly initialized non-null strings

Example fix

// before
var ignored = new string[3];
ignored[0] = "Id"; // [1] and [2] are null
var colInfo = new ColumnInformation { IgnoredColumnNames = ignored };

// after
var ignored = new[] { "Id" };
var colInfo = new ColumnInformation { IgnoredColumnNames = ignored };

Defensive patterns

Strategy: validation

Validate before calling

static string[] CleanNames(IEnumerable<string> names) => names?.Where(n => n != null).ToArray();
colInfo.IgnoredColumnNames = CleanNames(rawIgnored);

Type guard

static bool HasNoNulls(IEnumerable<string> names) => names == null || names.All(n => n != null);

Try / catch

try { var r = experiment.Execute(data, colInfo); }
catch (ArgumentException ex) when (ex.Message.Contains("Null column string")) { /* filter nulls from lists and retry */ }

Prevention

Use collection expressions or explicit initializers instead of new string[n]
Apply Where(n => n != null) when building name lists from external config
Enable nullable reference types so null string entries are flagged at compile time

When it happens

Trigger: Passing string[] or IEnumerable<string> containing null to ColumnInformation.CategoricalColumnNames / TextColumnNames / IgnoredColumnNames, e.g. from an array sized larger than the number of names filled in.

Common situations: new string[3] { "A", "B", null } patterns; building lists from possibly-null config values without filtering; LINQ Select over data with missing names yielding nulls.

Related errors

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/44d15bb2c7dc089e. Report an issue: GitHub.