How do I implement C# 14 extension block on a generic?

Question

I'm attempting to rewrite one of my extensions in the new block syntax. It's an extension on IEnumerable<T>:

public static class IEnumerableExtensions
{
    public static double StdDev<T>(this IEnumerable<T> list, Func<T, double?> values)
    {
        // ref: https://stackoverflow.com/questions/2253874/linq-equivalent-for-standard-deviation
        var mean = 0.0;
        var sum = 0.0;
        var stdDev = 0.0;
        var n = 0;

        foreach (var value in list.Select(values))
        {
            if (value is null)
                continue;

            n++;
            var delta = value.Value - mean;
            mean += delta / n;
            sum += delta * (value.Value - mean);
        }

        if (1 < n)
            stdDev = Math.Sqrt(sum / (n - 1));

        return stdDev;
    }
}

I was under the impression it would be as simple as:

public static class IEnumerableExtensions2
{
    extension(IEnumerable<T> list) // Bang! (1)
    {
        public double StdDev<T>(Func<T, double?> values)
        {
            // ref: https://stackoverflow.com/questions/2253874/linq-equivalent-for-standard-deviation
            var mean = 0.0;
            var sum = 0.0;
            var stdDev = 0.0;
            var n = 0;

            foreach (var value in list.Select(values)) // Bang! (2)
            {
                if (value is null)
                    continue;

                n++;
                var delta = value.Value - mean;
                mean += delta / n;
                sum += delta * (value.Value - mean);
            }

            if (1 < n)
                stdDev = Math.Sqrt(sum / (n - 1));

            return stdDev;
        }
    }
}

But I receive the errors:

The type or namespace name 'T' could not be found (are you missing a using directive or an assembly reference?)
'IEnumerable' does not contain a definition for 'Select' and the best extension method overload 'ImmutableArrayExtensions.Select<T, double?>(ImmutableArray, Func<T, double?>)' requires a receiver of type 'System.Collections.Immutable.ImmutableArray'

I'd like to know how I can include generics in extension blocks.

Try extension<T>(IEnumerable<T> list). See extension member blog — JonasH
– JonasH, Commented yesterday

Charlieface · Accepted Answer · 2025-11-21 11:13:06Z

6

As noted in the docs:

You need to declare the T generic parameter on the extension itself in order to make it in scope across the whole block

public static class IEnumerableExtensions2
{
    extension<T>(IEnumerable<T> list)
    {
        public double StdDev(Func<T, double?> values)
        {
            // ref: https://stackoverflow.com/questions/2253874/linq-equivalent-for-standard-deviation
            var mean = 0.0;
            var sum = 0.0;
            var n = 0;

            foreach (var value in list.Select(values))
            {
                if (value is null)
                    continue;

                n++;
                var delta = value.Value - mean;
                mean += delta / n;
                sum += delta * (value.Value - mean);
            }

            if (1 < n)
                stdDev = Math.Sqrt(sum / (n - 1));

            return stdDev;
        }
    }
}

On a side note: LINQ aggregations usually use the following logic when there are no items in source:

Nullable input: return null
Non-nullable: throw InvalidOperationException

So that would end up with

public static class IEnumerableExtensions2
{
    extension<T>(IEnumerable<T> list)
    {
        private (double sum, double n) StdDevImpl(IEnumerable<double> values)
        {
            // ref: https://stackoverflow.com/questions/2253874/linq-equivalent-for-standard-deviation
            var mean = 0.0;
            var sum = 0.0;
            var n = 0;

            foreach (var value in list.Select(values))
            {
                n++;
                var delta = value.Value - mean;
                mean += delta / n;
                sum += delta * (value.Value - mean);
            }

            return (sum, n);
        }

        public double StdDev(Func<T, double> values)
        {
            var (sum, n) = StdDevImpl(values);
            return n switch
            {
                0 => throw new InvalidOperationException("Sequence contains no elements"),
                1 => 0.0,
                _ => Math.Sqrt(sum / (n - 1)),
            };
        }

        public double? StdDev(Func<T, double?> values)
        {
            var (sum, n) = StdDevImpl(values.Where(n => n is not null).Select(n => n.GetValueOrDefault());
            return n switch
            {
                0 => null,
                1 => 0.0,
                _ => Math.Sqrt(sum / (n - 1)),
            };
        }
    }
}

edited 22 hours ago

answered yesterday

Charlieface

78.5k8 gold badges35 silver badges77 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Canyon yesterday

Thank you! I was skimming some of the getting started documentation but it appears there's not a mention of it yet

Jamiec 23 hours ago

Bit of an epic fail of docs, especially given as the example was right there in the whats new in c#14 page

Canyon 16 hours ago

PR merged on adding more information 👍

Jamiec 23 hours ago

If were going with standards of implementation of enumerable extensions the generic argument tends to be called TSource

Collectives™ on Stack Overflow

How do I implement C# 14 extension block on a generic?

1 Answer 1

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related