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Tag Archives: Aggregate

Obvious Extension Methods: IEnumerable Lazy DefaultIfEmpty

Is your Aggregate throwing an InvalidOperationException: Sequence contains no elements?
This line of code certainly would:
new string[]{}.Aggregate((ag, s) => ag + ", " + s)

Use this instead:
new string[]{}.DefaultIfEmpty("[NA]").Aggregate((ag, s) => ag + ", " + s)

I was writing some extension methods for things such as Aggregate where the only purpose was to pass in default values. That was not very smart, since you can always use DefaultIfEmpty() and pass your default value there. Really, what I needed was an overload of DefaultIfEmpty that takes a Func to get the default value, in case the default value is expensive (a database look-up, for example)

So here that is:

public static IEnumerable<TSource> DefaultIfEmpty<TSource>(this IEnumerable<TSource> source, Func<TSource> getDefaultValue)
{
	using (var enumerator = source.GetEnumerator())
	{
		if (enumerator.MoveNext())
		{
			do
			{
				yield return enumerator.Current;
			}
			while (enumerator.MoveNext());
		}
		else
			yield return getDefaultValue();
	}
}

Now you can say
new string[]{}.DefaultIfEmpty(() => goGetTheDefaultValue()).Aggregate((ag, s) => ag + ", " + s)

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Obvious Extension Method: IEnumerable MaxOrDefault and MinOrDefault

This one is even too obvious to comment on:

public static TResult MaxOrDefault(this IEnumerable source, Func selector, TResult defaultValue)
{
	return source.Select(selector).DefaultIfEmpty(defaultValue).Max();
}

public static TResult MaxOrDefault(this IEnumerable source, Func selector)
{
	return source.Select(selector).DefaultIfEmpty().Max();
}
		
public static TResult MinOrDefault(this IEnumerable source, Func selector, TResult defaultValue)
{
	return source.Select(selector).DefaultIfEmpty(defaultValue).Min();
}

public static TResult MinOrDefault(this IEnumerable source, Func selector)
{
	return source.Select(selector).DefaultIfEmpty().Min();
}

It just makes the code a little more succinct when trying to avoid the InvalidOperationException: Sequence contains no elements