dotnet/machinelearning · error · System.ArgumentException

Array lengths are mistmached

Error message

Array lengths are mistmached

What it means

Merge requires the retained and supplementary join-key arrays to have the same number of entries — one join column per side per key. When lengths differ it throws ArgumentException(Strings.MismatchedArrayLengths, nameof(retainedJoinColumnNames)); note the resource string reads 'Array lengths are mistmached' (typo included).

Source

Thrown at src/Microsoft.Data.Analysis/DataFrame.Join.cs:183

        /// <returns></returns> 
        public DataFrame Merge<TKey>(DataFrame other, string leftJoinColumn, string rightJoinColumn, string leftSuffix = "_left", string rightSuffix = "_right", JoinAlgorithm joinAlgorithm = JoinAlgorithm.Left)
        {
            return Merge(other, new[] { leftJoinColumn }, new[] { rightJoinColumn }, leftSuffix, rightSuffix, joinAlgorithm);
        }

        private static HashSet<long> Merge(DataFrame retainedDataFrame, DataFrame supplementaryDataFrame,
            string[] retainedJoinColumnNames, string[] supplemetaryJoinColumnNames,
            out PrimitiveDataFrameColumn<long> retainedRowIndices, out PrimitiveDataFrameColumn<long> supplementaryRowIndices,
            bool isInner = false, bool calculateIntersection = false)
        {
            if (retainedJoinColumnNames == null)
                throw new ArgumentNullException(nameof(retainedJoinColumnNames));

            if (supplemetaryJoinColumnNames == null)
                throw new ArgumentNullException(nameof(supplemetaryJoinColumnNames));

            if (retainedJoinColumnNames.Length != supplemetaryJoinColumnNames.Length)
                throw new ArgumentException(Strings.MismatchedArrayLengths, nameof(retainedJoinColumnNames));

            Dictionary<long, ICollection<long>> occurrences = GetOccurences(retainedDataFrame, supplementaryDataFrame,
                retainedJoinColumnNames, supplemetaryJoinColumnNames, out HashSet<long> supplementaryJoinColumnsNullIndices);

            return PerformMerging(retainedDataFrame, retainedJoinColumnNames, occurrences, supplementaryJoinColumnsNullIndices,
                out retainedRowIndices, out supplementaryRowIndices, isInner, calculateIntersection);
        }

        private static Dictionary<long, ICollection<long>> GetOccurences(DataFrame retainedDataFrame, DataFrame supplementaryDataFrame,
            string[] retainedJoinColumnNames, string[] supplemetaryJoinColumnNames, out HashSet<long> supplementaryJoinColumnsNullIndices)
        {
            supplementaryJoinColumnsNullIndices = new HashSet<long>();

            // Get occurrences of values in columns used for join in the retained and supplementary dataframes

            Dictionary<long, ICollection<long>> occurrences = null;
            Dictionary<long, long> retainedIndicesReverseMapping = null;

View on GitHub (pinned to 7b76e69cf9)

Solutions

1. Make both arrays the same length, pairing columns positionally for each join key
2. Include every composite-key column on both sides
3. Validate lengths and contents exist in each DataFrame before merging
4. Catch ArgumentException and report which join specification was mismatched

Example fix

// before
df.Merge(other, new[] { "Id" }, new[] { "Id", "TenantId" });
// after
df.Merge(other, new[] { "Id", "TenantId" }, new[] { "Id", "TenantId" });

if (leftKeys == null || rightKeys == null || leftKeys.Length != rightKeys.Length)
    throw new InvalidOperationException("Join key arrays must have equal length");

try { return df.Merge(other, leftKeys, rightKeys); } catch (ArgumentException ex) when (ex.Message.Contains("mistmached")) { throw new InvalidOperationException("Composite join keys must be paired on both sides", ex); }

When it happens

Trigger: Calling Merge with retainedJoinColumnNames.Length != supplemetaryJoinColumnNames.Length, e.g. df.Merge(other, new[]{"Id"}, new[]{"Id","TenantId"}).

Common situations: Joining composite keys where one side lists fewer columns; building the key arrays from different config sources that drifted; renaming a column on one side so it was dropped from one array.

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

• Exception of type 'System.ArgumentException' was thrown.
• Expected either {0} or {1} to be provided
• Expected a seekable stream
• Decimal separator cannot match the column separator
• Value cannot be null. (Parameter 'retainedJoinColumnNames')

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/cb7e3350658475c3. Report an issue: GitHub.