Fast String Concatenation (better than StringBuilder when it can apply)

StringConcatenationExtensions.cs

using System;
using System.Collections.Generic;

namespace System;

public static class StringConcatenationExtensions
{
    /// <summary>
    /// Concatenates strings with manamal allocations and good performance.
    /// (only the end result string is allocated)
    /// </summary>
    /// <param name="strings"></param>
    /// <returns></returns>
    public unsafe static string Concatenate(this string[] strings)
    {
        int size = 0;
        for (int i = 0; i < strings.Length; i++)
        {
            var str = strings[i];
            if (str == null)
                continue;

            size += str.Length;
        }

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

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

            int pos = 0;
            for (int i = 0; i < strings.Length; i++)
            {
                var str = strings[i];
                if (str == null)
                    continue;

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

        return result;
    }
}