What causes "Column lengths are mismatched" in dotnet/machinelearning?

Align the columns: filter/reindex both to the same row set before the operation

dotnet/machinelearning · error · ArgumentException

Column lengths are mismatched

Error message

Column lengths are mismatched

What it means

Binary operations between two PrimitiveDataFrameColumn instances require both columns to have the same Length. HandleOperationImplementation<U> validates column.Length == Length before dispatching the typed operation and throws ArgumentException (message 'Column lengths are mismatched') with paramName 'column' when they differ.

Source

Thrown at src/Microsoft.Data.Analysis/PrimitiveDataFrameColumn.cs:957

        public override PrimitiveDataFrameColumn<bool> ElementwiseIsNotNull()
        {
            var ret = new BooleanDataFrameColumn(Name, Length);

            for (long i = 0; i < Length; i++)
            {
                ret[i] = IsValid(i);
            }

            return ret;
        }

        internal DataFrameColumn HandleOperationImplementation<U>(BinaryOperation operation, PrimitiveDataFrameColumn<U> column, bool inPlace)
            where U : unmanaged
        {
            if (column.Length != Length)
            {
                throw new ArgumentException(Strings.MismatchedColumnLengths, nameof(column));
            }
            switch (typeof(T))
            {
                case Type boolType when boolType == typeof(bool):
                    if (typeof(U) == typeof(bool))
                    {
                        PrimitiveDataFrameColumn<U> primitiveColumn = this as PrimitiveDataFrameColumn<U>;
                        var newColumn = inPlace ? primitiveColumn : primitiveColumn.Clone();
                        newColumn._columnContainer.HandleOperation(operation, column._columnContainer);
                        return newColumn;
                    }
                    throw new NotSupportedException();
                case Type decimalType when decimalType == typeof(decimal):
                    if (typeof(U) == typeof(bool))
                    {
                        throw new NotSupportedException();
                    }
                    if (typeof(U) == typeof(T))

View on GitHub (pinned to 7b76e69cf9)

Solutions

Align the columns: filter/reindex both to the same row set before the operation
Truncate to the shorter length explicitly if that is the intended semantics
Re-create the second column from the same source rows as the first
Check col1.Length == col2.Length before calling the operation and handle mismatches

Example fix

// before
var sum = colA + colB; // colA.Length=100, colB.Length=90 -> ArgumentException
// after
if (colA.Length != colB.Length)
    throw new InvalidOperationException($"Column length mismatch: {colA.Length} vs {colB.Length}");
var sum = colA + colB;

Defensive patterns

Strategy: validation

Validate before calling

if (left.Length != right.Length)
    throw new InvalidOperationException($"Column lengths mismatched: {left.Length} vs {right.Length}");

Type guard

bool SameLength<T,U>(PrimitiveDataFrameColumn<T> a, PrimitiveDataFrameColumn<U> b) => a.Length == b.Length;

Try / catch

try
{
    var result = left.Add(right);
}
catch (ArgumentException ex) when (ex.Message == Strings.MismatchedColumnLengths)
{
    // align/reindex columns and retry
}

Prevention

Derive both columns from the same filtered/aligned DataFrame
Assert column lengths equal before vectorized operations
Avoid mutating one column's length independently of its siblings

When it happens

Trigger: Any arithmetic/comparison binary operation (Add, Subtract, Multiply, Divide, comparisons via BinaryOperation) between two columns of different lengths, e.g. col1 + col2 where col1.Length != col2.Length.

Common situations: Joining/filtering one column but not the other before a vectorized operation; constructing columns from arrays of different sizes; rows dropped from one column independently.

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

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/35e4f8ef7e2da56a. Report an issue: GitHub.