dotnet/machinelearning · error · ArgumentNullException

Value cannot be null. (Parameter 'getter') Getter of type '{

Error message

Value cannot be null. (Parameter 'getter')
Getter of type '{typeof(TValue)}' expected, but {getter.GetType()} found

What it means

CheckGetter<TValue> verifies each annotation getter is a ValueGetter<TValue> matching the declared annotation column's RawType; if the delegate exists but has the wrong type, it throws ArgumentNullException naming 'getter' with a message showing the expected TValue and the actual delegate type. This is a type-conformance check even though it throws ArgumentNullException.

Source

Thrown at src/Microsoft.ML.DataView/DataViewSchema.cs:215

                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)
            {
                if (column.Index >= _getters.Length)
                    throw new ArgumentException(nameof(column));
                var typedGetter = _getters[column.Index] as ValueGetter<TValue>;
                if (typedGetter == null)
                {
                    Debug.Assert(_getters[column.Index] != null);
                    throw new InvalidOperationException($"Invalid call to '{nameof(GetGetter)}'");
                }
                return typedGetter;
            }

View on GitHub (pinned to 7b76e69cf9)

Solutions

Align each getter to ValueGetter<TValue> where TValue equals the annotation column's Type.RawType.
Cast/construct the delegate explicitly as (ValueGetter<TValue>)((ref TValue value) => ...) so the compiler enforces the type.
Fix the annotation schema declaration to match the getter type actually provided.
Add a unit test constructing the Annotations to catch type drift early.

Example fix

// before
Delegate g = (ValueGetter<float>)(ref float v) => v = 1f; // annotation slot is RawType double
new Annotations(schema, new Delegate[] { g });
// after
Delegate g = (ValueGetter<double>)(ref double v) => v = 1.0; // matches RawType
new Annotations(schema, new Delegate[] { g });

Defensive patterns

Strategy: type-guard

Validate before calling

bool ok = getter is ValueGetter<double> && schema[0].Type.RawType == typeof(double);

Type guard

bool IsTypedGetter<TValue>(Delegate g) => g is ValueGetter<TValue>;

Prevention

Declare getters as explicit ValueGetter<TValue> so the compiler catches mismatches
Keep annotation schema types and getter types in sync; test Annotations construction

When it happens

Trigger: Passing a Delegate to Annotations whose runtime type is not ValueGetter<TValue> for the corresponding schema slot's RawType — e.g. a ValueGetter<float> supplied where the annotation type is double, or a plain lambda typed as Action/Func instead of ValueGetter.

Common situations: Mismatched annotation type declarations vs. getter implementations; refactoring that changed TValue of the getter but not the annotation schema; boxing conversions (int getter for uint annotation).

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/aa6bb02406bc9aeb. Report an issue: GitHub.