dotnet/machinelearning · error · ArgumentNullException

Value cannot be null. (Parameter 'name')

Error message

Value cannot be null. (Parameter 'name')

What it means

DataViewSchema's string indexer looks up a column by name using an internal name map. Null or empty names are rejected with ArgumentNullException('name') before the lookup, since a null/empty column name can never exist in the schema.

Solutions

1. Check string.IsNullOrEmpty(name) before indexing the schema
2. Use GetColumnOrNull(name, out col) to safely probe for a column without throwing
3. Fix the upstream source that produced a null/empty column name

Example fix

// before
var col = schema[name];
// after
if (!schema.GetColumnOrNull(name, out var col))
    throw new ArgumentException($"Column '{name}' not found");

if (string.IsNullOrEmpty(name)) throw new ArgumentException("Column name must be non-empty");
var col = schema[name];

try { var col = schema[name]; }
catch (ArgumentNullException ex) when (ex.ParamName == "name")
{ /* null/empty name — fix source of the name */ }

When it happens

Trigger: Accessing schema[name] where name is null or the empty string, often from an unvalidated variable or an API that returned a null column name.

Common situations: Copy-pasting a variable name into the indexer instead of the literal; upstream code returning null column names; deserialization producing null metadata names.

Related errors

• Column ' ' not found (Parameter 'name')
• Value cannot be null. (Parameter 'type')
• argument should not be null.
• ArgumentNullException
• ArgumentNullException(nameof(column))

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/477037b3b24b2398. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.ML.DataView/DataViewSchema.cs:38

    {
        private readonly Column[] _columns;
        private readonly Dictionary<string, int> _nameMap;

        /// <summary>
        /// Number of columns in the schema.
        /// </summary>
        public int Count => _columns.Length;

        /// <summary>
        /// Get the column by name. Throws an exception if such column does not exist.
        /// Note that if multiple columns exist with the same name, the one with the biggest index is returned.
        /// The other columns are considered 'hidden', and only accessible by their index.
        /// </summary>
        public Column this[string name]
        {
            get
            {
                if (string.IsNullOrEmpty(name)) throw new ArgumentNullException(nameof(name));
                if (!_nameMap.TryGetValue(name, out int col))
                    throw new ArgumentOutOfRangeException(nameof(name), $"Column '{name}' not found");
                return _columns[col];
            }
        }

        /// <summary>
        /// Get the column by index.
        /// </summary>
        public Column this[int columnIndex]
        {
            get
            {
                if (!(0 <= columnIndex && columnIndex < _columns.Length))
                    throw new ArgumentOutOfRangeException(nameof(columnIndex));
                return _columns[columnIndex];
            }
        }

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