dotnet/machinelearning · error · ArgumentException

MismatchedColumnLengths

Error message

MismatchedColumnLengths

What it means

StringDataFrameColumn.Add (and its arithmetic family) requires both columns to have the same Length; element i is combined with element i. When Length != column.Length it throws ArgumentException with the message Strings.MismatchedColumnLengths ('MismatchedColumnLengths'). The library has no broadcasting or alignment for binary column operations.

Source

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

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

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;

namespace Microsoft.Data.Analysis
{
    public partial class StringDataFrameColumn : DataFrameColumn
    {
        /// <inheritdoc/>
        public override DataFrameColumn Add(DataFrameColumn column, bool inPlace = false)
        {
            if (Length != column.Length)
            {
                throw new ArgumentException(Strings.MismatchedColumnLengths, nameof(column));
            }
            StringDataFrameColumn ret = inPlace ? this : Clone();
            for (long i = 0; i < Length; i++)
            {
                ret[i] += column[i].ToString();
            }
            return ret;
        }

        public static StringDataFrameColumn operator +(StringDataFrameColumn column, string value)
        {
            return column.Add(value);
        }

        public static StringDataFrameColumn operator +(string value, StringDataFrameColumn column)
        {
            return Add(value, column);
        }

View on GitHub (pinned to 7b76e69cf9)

Solutions

1. Ensure both columns have the same Length before adding: filter/pad/truncate so counts match
2. Verify the intended operand — if you meant to append a constant, use the string overload (Add(string value)) instead of a column overload
3. Re-derive both columns from the same filtered DataFrame so row counts stay in sync
4. Check for null/missing rows removed on one side only; rebuild with an index-based join

Example fix

// before
var filtered = df["A"].ElementwiseLessThan(10); // shorter after use elsewhere
var result = strColumn.Add(otherStrColumn); // Lengths 100 vs 90 -> ArgumentException
// after
if (strColumn.Length != otherStrColumn.Length)
    throw new InvalidOperationException("Align columns before Add");
var result = strColumn.Add(otherStrColumn);

if (left.Length != right.Length)
    throw new InvalidOperationException($"Cannot Add: lengths differ ({left.Length} vs {right.Length})");

bool SameLength(DataFrameColumn a, DataFrameColumn b) => a.Length == b.Length;

try { var result = strColumn.Add(otherColumn); }
catch (ArgumentException ex) when (ex.Message.Contains("MismatchedColumnLengths")) {
    // align lengths: filter both columns from the same source or pad/truncate
}

When it happens

Trigger: Calling stringColumn.Add(otherColumn) — including the + operator and Add(column, inPlace:true) — where the two columns have different row counts, e.g. Length=100 vs Length=90 after a filter/join on only one side.

Common situations: Filtering one column independently before concatenating; misaligned results from a left join; appending rows to one column only; mixing columns from DataFrames of different sizes.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• Column lengths are mismatched
• Expected column to hold values of type {0}
• MismatchedColumnLengths
• Exception of type 'System.ArgumentException' was thrown.
• Parameter must not be null, empty, or whitespace

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/36e70145fe9f257b. Report an issue: GitHub.