dotnet/machinelearning · error · ArgumentException

Strings.MismatchedColumnLengths

Error message

Strings.MismatchedColumnLengths

What it means

When inserting a column into a non-empty DataFrameColumnCollection, the column's Length must equal the collection's RowCount, since all columns of a DataFrame must have the same number of rows. A length mismatch throws ArgumentException(Strings.MismatchedColumnLengths, nameof(column)) from InsertItem.

Source

Thrown at src/Microsoft.Data.Analysis/DataFrameColumnCollection.cs:82

            where T : unmanaged
        {
            DataFrameColumn newColumn = new PrimitiveDataFrameColumn<T>(columnName, column);
            Insert(columnIndex, newColumn); // calls InsertItem internally
        }

        protected override void InsertItem(int columnIndex, DataFrameColumn column)
        {
            column = column ?? throw new ArgumentNullException(nameof(column));

            if (Count == 0)
            {
                //change RowCount on inserting first row to dataframe
                RowCount = column.Length;
            }
            else if (column.Length != RowCount)
            {
                //check all columns in the dataframe have the same length (amount of rows)
                throw new ArgumentException(Strings.MismatchedColumnLengths, nameof(column));
            }

            if (_columnNameToIndexDictionary.ContainsKey(column.Name))
            {
                throw new ArgumentException(string.Format(Strings.DuplicateColumnName, column.Name), nameof(column));
            }

            column.AddOwner(this);

            RowCount = column.Length;

            _columnNameToIndexDictionary[column.Name] = columnIndex;
            for (int i = columnIndex; i < Count; i++)
            {
                _columnNameToIndexDictionary[this[i].Name]++;
            }
            base.InsertItem(columnIndex, column);
            ColumnsChanged?.Invoke();

View on GitHub (pinned to 7b76e69cf9)

Solutions

  1. Resize the new column to match df.Columns.RowCount before inserting (recreate PrimitiveDataFrameColumn with the right length or pad/truncate data).
  2. Check column.Length == df.Columns.RowCount before inserting and fail fast with a clear message.
  3. If rows are genuinely missing, pad with nulls/default values or fix the upstream data load.
  4. Create all columns from same-length sources when assembling a DataFrame.

Example fix

// before
df.Columns.Insert(0, new PrimitiveDataFrameColumn<int>("Flag", shortData)); // length mismatch
// after
if (shortData.Length != df.Columns.RowCount)
    throw new InvalidOperationException($"Column has {shortData.Length} rows, expected {df.Columns.RowCount}");
df.Columns.Insert(0, new PrimitiveDataFrameColumn<int>("Flag", df.Columns.RowCount));
Defensive patterns

Strategy: validation

Validate before calling

if (column.Length != df.Columns.RowCount)
    throw new InvalidOperationException($"Column '{column.Name}' has {column.Length} rows; DataFrame has {df.Columns.RowCount}.");

Try / catch

try { df.Columns.Insert(idx, column); }
catch (ArgumentException ex) when (ex.ParamName == "column")
{
    // pad/truncate the column to RowCount and retry, or surface a data-loading error
}

Prevention

When it happens

Trigger: df.Columns.Insert(i, column) or InsertColumn with a column whose Length differs from the existing RowCount — e.g. adding a 3-row column to a DataFrame whose columns have 100 rows, or inserting into an empty-name-collision-free but differently sized collection.

Common situations: Merging columns from DataFrames loaded from different files; building a DataFrame column-by-column where one source array is shorter; off-by-one or filtered series added to an existing table.

Related errors

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/08147843c23aa8f7. Report an issue: GitHub.