dotnet/machinelearning · error · ArgumentException

MismatchedColumnLengths

Error message

MismatchedColumnLengths

What it means

DataFrame.Add<T>(IReadOnlyList<T> values, bool inPlace) throws ArgumentException(Strings.MismatchedColumnLengths) when the values list length differs from the DataFrame's column count. The list must supply one operand per column for the element-wise addition.

Source

Thrown at src/Microsoft.Data.Analysis/DataFrame.BinaryOperations.cs:20

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

// Generated from DataFrameBinaryOperations.tt. Do not modify directly

using System;
using System.Collections.Generic;

namespace Microsoft.Data.Analysis
{
    public partial class DataFrame
    {
        public DataFrame Add<T>(IReadOnlyList<T> values, bool inPlace = false)
            where T : unmanaged
        {
            if (values.Count != Columns.Count)
            {
                throw new ArgumentException(Strings.MismatchedColumnLengths, nameof(values));
            }
            DataFrame retDataFrame = inPlace ? this : new DataFrame();

            for (int i = 0; i < Columns.Count; i++)
            {
                DataFrameColumn baseColumn = _columnCollection[i];
                DataFrameColumn newColumn = baseColumn.Add(values[i], inPlace);
                if (inPlace)
                    retDataFrame.Columns[i] = newColumn;
                else
                    retDataFrame.Columns.Insert(i, newColumn);
            }
            return retDataFrame;
        }
        /// <summary>
        /// Performs an element-wise addition on each column
        /// </summary>
        public DataFrame Add<T>(T value, bool inPlace = false)

View on GitHub (pinned to 7b76e69cf9)

Solutions

1. Check values.Count equals df.Columns.Count before calling Add and trim or extend the list to match.
2. Build the values list dynamically from df.Columns.Count instead of a hardcoded literal.
3. If column count legitimately differs, construct a new DataFrame with the intended columns and add row-wise/element-wise explicitly.

Example fix

// before
df.Add(new int[] { 1, 2, 3 }); // df has 5 columns -> ArgumentException
// after
if (values.Length == df.Columns.Count)
{
    df.Add(values);
}

if (values.Count != df.Columns.Count)
    throw new ArgumentException($"Expected {df.Columns.Count} values, got {values.Count}");

static bool MatchesColumnCount<T>(DataFrame df, IReadOnlyList<T> values) => values.Count == df.Columns.Count;

try
{
    df.Add(values);
}
catch (ArgumentException ex) when (ex.Message.Contains("MismatchedColumnLengths"))
{
    // rebuild the values list from df.Columns.Count and retry
}

When it happens

Trigger: Calling df.Add(values) where values.Count != df.Columns.Count; e.g. adding a 3-element list to a 5-column DataFrame.

Common situations: Building the values list from a separate schema/row of data; columns added or dropped after the list was created; hardcoded arrays not matching the DataFrame schema.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Parameter must not be null, empty, or whitespace
• Value cannot be null. (Parameter 'idColumns')
• Must provide at least 1 ID column
• Must provide at least 1 value column when specifying value c
• Columns cannot exist in both idColumns and valueColumns

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/58a2f292df508804. Report an issue: GitHub.