Fast String Concatenation

ConcatenateExtensions.cs

public static class ConcatenateExtensions
{
    public static unsafe string Concatenate(this IReadOnlyList<string?> strings)
    {
        int size = 0;
        for (int i = 0; i < strings.Count; i++)
        {
            string? str = strings[i];
            if (str == null)
            {
                continue;
            }

            size += str.Length;
        }

        // We precallocate the string result,
        string result = new(' ', size);

        fixed (char* ptr = result)
        {
            var span = new Span<char>(ptr, size);

            int pos = 0;
            for (int i = 0; i < strings.Count; i++)
            {
                string? str = strings[i];
                if (str == null)
                {
                    continue;
                }

                int len = str.Length;
                str.CopyTo(span.Slice(pos, len));
                pos += len;
            }
        }

        return result;
    }
}