dotnet/machinelearning · error · ArgumentNullException

Value cannot be null. (Parameter 'getter') Delegate at index

Error message

Value cannot be null. (Parameter 'getter')
Delegate at index '{i}' of getters was null.

What it means

The Annotations constructor validates every delegate in the getters array: if any element is null it throws ArgumentNullException naming 'getter' with a message identifying the offending index i. Annotations are backed by one typed getter per annotation slot, so a null getter would break all subsequent value access.

Source

Thrown at src/Microsoft.ML.DataView/DataViewSchema.cs:204

            /// <summary>
            /// Create an annotations row by supplying the schema columns and the getter delegates for all the values.
            /// </summary>
            /// <remarks>
            /// Note: The <paramref name="getters"/> array is owned by this <see cref="Annotations"/> instance.
            /// </remarks>
            internal Annotations(DataViewSchema schema, Delegate[] getters)
            {
                Debug.Assert(schema != null);
                Debug.Assert(getters != null);

                Debug.Assert(schema.Count == getters.Length);
                // Check all getters.
                for (int i = 0; i < schema.Count; i++)
                {
                    var getter = getters[i];
                    if (getter == null)
                        throw new ArgumentNullException(nameof(getter), $"Delegate at index '{i}' of {nameof(getters)} was null.");
                    Utils.MarshalActionInvoke(CheckGetter<int>, schema[i].Type.RawType, getter);
                }
                Schema = schema;
                _getters = getters;
            }

            private void CheckGetter<TValue>(Delegate getter)
            {
                var typedGetter = getter as ValueGetter<TValue>;
                if (typedGetter == null)
                    throw new ArgumentNullException(nameof(getter), $"Getter of type '{typeof(TValue)}' expected, but {getter.GetType()} found");
            }

            /// <summary>
            /// Get a getter delegate for one value of the annotations row.
            /// </summary>
            public ValueGetter<TValue> GetGetter<TValue>(DataViewSchema.Column column)
            {

View on GitHub (pinned to 7b76e69cf9)

Solutions

1. Ensure every element of the getters array is a non-null typed delegate (ValueGetter<TValue>) before calling the constructor.
2. Add your own null check with a clearer message at the point you build the array.
3. Fix the conditional/loop logic that leaves an entry unassigned.
4. Only pass arrays whose length equals schema.Count (asserted by the constructor) and fully populated.

Example fix

// before
var getters = new Delegate[] { GetNameGetter(), null, GetCountGetter() };
var annotations = new Annotations(schema, getters);
// after
var getters = new Delegate[] { GetNameGetter(), GetWeightsGetter(), GetCountGetter() };
if (getters.Any(g => g == null)) throw new InvalidOperationException("All annotation getters must be provided");
var annotations = new Annotations(schema, getters);

if (getters == null || getters.Length != schema.Count || getters.Any(g => g == null))
    throw new InvalidOperationException("Every annotation slot needs a non-null getter");

bool GettersComplete(Delegate[] g) => g != null && g.All(x => x != null);

try { var ann = new Annotations(schema, getters); } catch (ArgumentNullException ex) { /* ex.Message names the null index */ }

When it happens

Trigger: Building an Annotations object with a getters array that contains a null element — e.g. constructing delegates in a loop with conditional assignment, or a factory returning null for one annotation kind.

Common situations: Custom trainers/transforms emitting annotation getters (slot names, per-slot metrics) where one getter function returned null; parallel delegate construction where an entry is skipped.

Related errors

• Value cannot be null. (Parameter 'getter') Getter of type '{
• Value cannot be null. (Parameter 'idColumns')
• ArgumentNullException(nameof(column))
• null
• value

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/d5d12710fe49d2bc. Report an issue: GitHub.