Using Enumerable.Aggregate(…) Method over an empty sequence

To concatenate a list of strings, use the string.Join method.

The Aggregate function doesn’t work with empty collections. It requires a binary accumulate function and it needs an item in the collection to pass to the binary function as a seed value.

However, there is an overload of Aggregate:

public static TResult Aggregate<TSource, TAccumulate, TResult>(
    this IEnumerable<TSource> source,
    TAccumulate seed,
    Func<TAccumulate, TSource, TAccumulate> func,
    Func<TAccumulate, TResult> resultSelector
)

This overload allows you to specify a seed value. If a seed value is specified, it will also be used as the result if the collection is empty.

EDIT: If you’d really want to use Aggregate, you can do it this way:

sequence.Aggregate(string.Empty, (x, y) => x == string.Empty ? y : x + Separator + y)

Or this way by using StringBuilder:

sequence.Aggregate(new StringBuilder(), (sb, x) => (sb.Length == 0 ? sb : sb.Append(Separator)).Append(x)).ToString()