dotnet/machinelearning · error · ArgumentException

Strings.DuplicateColumnName (formatted with column.Name)

Error message

Strings.DuplicateColumnName (formatted with column.Name)

What it means

When replacing a column, the new name must not collide with any other existing column. If the name is found in the name-to-index dictionary at a different index, ArgumentException(Strings.DuplicateColumnName, column.Name) is thrown. Replacing in place with the same name is allowed.

Source

Thrown at src/Microsoft.Data.Analysis/DataFrameColumnCollection.cs:113

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

        protected override void SetItem(int columnIndex, DataFrameColumn column)
        {
            column = column ?? throw new ArgumentNullException(nameof(column));
            if (RowCount > 0 && column.Length != RowCount)
            {
                throw new ArgumentException(Strings.MismatchedColumnLengths, nameof(column));
            }
            bool existingColumn = _columnNameToIndexDictionary.TryGetValue(column.Name, out int existingColumnIndex);
            if (existingColumn && existingColumnIndex != columnIndex)
            {
                throw new ArgumentException(string.Format(Strings.DuplicateColumnName, column.Name), nameof(column));
            }

            _columnNameToIndexDictionary.Remove(this[columnIndex].Name);
            _columnNameToIndexDictionary[column.Name] = columnIndex;

            this[columnIndex].RemoveOwner(this);
            base.SetItem(columnIndex, column);

            ColumnsChanged?.Invoke();
        }

        protected override void RemoveItem(int columnIndex)
        {
            _columnNameToIndexDictionary.Remove(this[columnIndex].Name);
            for (int i = columnIndex + 1; i < Count; i++)
            {
                _columnNameToIndexDictionary[this[i].Name]--;
            }

View on GitHub (pinned to 7b76e69cf9)

Solutions

1. Choose a unique replacement name before assigning.
2. Remove the conflicting column first, then set the new one.
3. Reuse the existing column's name (same-name in-place replacement) if a rename is not intended.

Example fix

// before
df.Columns[0] = new Int32DataFrameColumn("B", df.Rows.Count); // 'B' is column 1
// after
df.Columns[0] = new Int32DataFrameColumn("A_unique", df.Rows.Count);

var existingIdx = df.Columns.TryGetColumnIndex(newColumn.Name, out int idx);
if (existingIdx && idx != index)
    throw new InvalidOperationException($"Name '{newColumn.Name}' belongs to column {idx}");
df.Columns[index] = newColumn;

bool CanReplaceAt(DataFrameColumn c, int index, DataFrame df) =>
    !df.Columns.TryGetColumnIndex(c.Name, out int i) || i == index;

try
{
    df.Columns[index] = column;
}
catch (ArgumentException ex) when (ex.Message.Contains("already exists") || ex.Message.Contains("Duplicate"))
{
    // pick a unique name and retry
}

When it happens

Trigger: df.Columns[i] = column where column.Name matches another column j != i in the collection.

Common situations: Renaming a column via replacement to a name already used by a sibling column, bulk column-swap code that reuses names, template columns copied without renaming.

Related errors

• string.Format(Strings.DuplicateColumnName, column.Name)
• Value name '{0}' matches an existing column name
• {0} and {1} must be different
• Strings.InvalidColumnName (formatted with columnName)
• Strings.BadColumnCast (formatted with column.DataType, typeo

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/08d6c739022cc26a. Report an issue: GitHub.