dotnet/machinelearning · error · ArgumentNullException

name

Error message

name

What it means

The strongly-typed Builder.Add<TValue> validates that the column name is non-null and non-empty before adding the annotation/column entry; an empty string or null name is rejected with ArgumentNullException 'name'. Every column and annotation must have an identifying name.

Source

Thrown at src/Microsoft.ML.DataView/DataViewSchema.cs:298

                    {
                        if (selector(column.Name))
                            _items.Add((column.Name, column.Type, annotations.GetGetterInternal(column.Index), column.Annotations));
                    }
                }

                /// <summary>
                /// Add one annotation column, strongly-typed version.
                /// </summary>
                /// <typeparam name="TValue">The type of the value.</typeparam>
                /// <param name="name">The annotation name.</param>
                /// <param name="type">The annotation type.</param>
                /// <param name="getter">The getter delegate.</param>
                /// <param name="annotations">Annotations of the input column. Note that annotations on an annotation column is somewhat rare
                /// except for certain types (for example, slot names for a vector, key values for something of key type).</param>
                public void Add<TValue>(string name, DataViewType type, ValueGetter<TValue> getter, Annotations annotations = null)
                {
                    if (string.IsNullOrEmpty(name))
                        throw new ArgumentNullException(nameof(name));
                    if (type == null)
                        throw new ArgumentNullException(nameof(type));
                    if (getter == null)
                        throw new ArgumentNullException(nameof(getter));
                    if (type.RawType != typeof(TValue))
                        throw new ArgumentException($"{nameof(type)}.{nameof(type.RawType)} must be of type '{typeof(TValue).FullName}'.", nameof(type));

                    _items.Add((name, type, getter, annotations));
                }

                /// <summary>
                /// Add one annotation column, weakly-typed version.
                /// </summary>
                /// <param name="name">The annotation name.</param>
                /// <param name="type">The annotation type.</param>
                /// <param name="getter">The getter delegate that provides the value. Note that the type of the getter is still checked
                /// inside this method.</param>
                /// <param name="annotations">Annotations of the input column. Note that annotations on an annotation column is somewhat rare

View on GitHub (pinned to 7b76e69cf9)

Solutions

1. Validate the name string before calling Add (non-null, non-empty)
2. Default to a meaningful name (e.g. the input column name) when the source value is blank
3. Throw your own descriptive error earlier so the user sees which config field was blank

Example fix

// before
builder.Add(name, type, getter);
// after
if (string.IsNullOrEmpty(name))
    throw new InvalidOperationException("Column name must be provided");
builder.Add(name, type, getter);

if (string.IsNullOrEmpty(name))
    throw new ArgumentException("Column name required", nameof(name));
builder.Add(name, type, getter);

try { builder.Add(name, type, getter); }
catch (ArgumentNullException e) when (e.ParamName == "name")
{
    builder.Add(fallbackName ?? "Column0", type, getter);
}

When it happens

Trigger: Passing string.Empty as a name; passing a name variable that came from a failed lookup or split operation and is null; building output schema in a custom transform with placeholder names never filled in.

Common situations: Dynamically generated column names from feature/label configuration where the user left the setting blank; constructing slot-name or key-value annotations with empty kind strings.

Related errors

• type
• selector
• getter
• The provided model file {filePath} doesn't exist.
• Start must be called on a ModelLoader before it can be used.

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/25e3f43af25d3fd4. Report an issue: GitHub.