dotnet/machinelearning · error · ArgumentException

Strings.BadColumnCast (formatted with column.DataType, typeo

Error message

Strings.BadColumnCast (formatted with column.DataType, typeof(string))

What it means

GetArrowStringColumn returns the named column as an ArrowStringDataFrameColumn or throws ArgumentException(Strings.BadColumnCast, column.DataType, typeof(string)) if it is a different column kind (commonly a StringDataFrameColumn).

Source

Thrown at src/Microsoft.Data.Analysis/DataFrameColumnCollection.cs:257

            throw new ArgumentException(string.Format(Strings.BadColumnCast, column.DataType, typeof(DateTime)));
        }

        /// <summary>
        /// Gets the <see cref="ArrowStringDataFrameColumn"/> with the specified <paramref name="name"/>.
        /// </summary>
        /// <param name="name">The name of the column</param>
        /// <returns><see cref="ArrowStringDataFrameColumn"/>.</returns>
        /// <exception cref="ArgumentException">A column named <paramref name="name"/> cannot be found, or if the column's type doesn't match.</exception>
        public ArrowStringDataFrameColumn GetArrowStringColumn(string name)
        {
            DataFrameColumn column = this[name];
            if (column is ArrowStringDataFrameColumn ret)
            {
                return ret;
            }

            throw new ArgumentException(string.Format(Strings.BadColumnCast, column.DataType, typeof(string)));
        }

        /// <summary>
        /// Gets the <see cref="StringDataFrameColumn"/> with the specified <paramref name="name"/>.
        /// </summary>
        /// <param name="name">The name of the column</param>
        /// <returns><see cref="StringDataFrameColumn"/>.</returns>
        /// <exception cref="ArgumentException">A column named <paramref name="name"/> cannot be found, or if the column's type doesn't match.</exception>
        public StringDataFrameColumn GetStringColumn(string name)
        {
            DataFrameColumn column = this[name];
            if (column is StringDataFrameColumn ret)
            {
                return ret;
            }

            throw new ArgumentException(string.Format(Strings.BadColumnCast, column.DataType, typeof(string)));
        }

View on GitHub (pinned to 7b76e69cf9)

Solutions

Check whether the column 'is ArrowStringDataFrameColumn' before calling.
If a StringDataFrameColumn is what you have, call GetStringColumn instead, or convert to ArrowStringDataFrameColumn.
Normalize string column storage at load time so accessors match.

Example fix

// before
var s = df.Columns.GetArrowStringColumn("Name"); // Name is StringDataFrameColumn
// after
var s = df.Columns["Name"] is ArrowStringDataFrameColumn arrow
    ? arrow
    : df.Columns.GetStringColumn("Name");

Defensive patterns

Strategy: type-guard

Validate before calling

var c = df.Columns["Name"];
var s = c is ArrowStringDataFrameColumn
    ? df.Columns.GetArrowStringColumn("Name")
    : throw new InvalidOperationException($"'Name' is {c.DataType}, not arrow string");

Type guard

bool IsArrowString(DataFrameColumn c) => c is ArrowStringDataFrameColumn;

Try / catch

try
{
    var s = df.Columns.GetArrowStringColumn(name);
}
catch (ArgumentException)
{
    // fall back to GetStringColumn or convert storage
}

Prevention

Know which ingestion path produces ArrowString vs String columns.
Normalize string column storage right after loading.
Use pattern matching instead of assuming string column kind.

When it happens

Trigger: df.Columns.GetArrowStringColumn("s") where the column is a StringDataFrameColumn or another non-arrow string storage.

Common situations: Columns built in memory as StringDataFrameColumn but fetched as ArrowString, data read from formats that map strings differently (CSV vs Arrow files).

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/7e5137f9bcb97a0f. Report an issue: GitHub.