AllocateUninitializedString.cs

AllocateUninitializedString.cs

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

public class Program
{
    static ref byte GetRawData(object obj) => 
        ref Unsafe.As<RawData>(obj).Data;

    [StructLayout(LayoutKind.Sequential)]
    class RawData { public byte Data; }

    static ref IntPtr GetMethodTable(object obj) => 
        ref Unsafe.Add(ref Unsafe.As<byte, IntPtr>(ref GetRawData(obj)), -1);

    static readonly IntPtr StringMT = GetMethodTable("string");

    static string AllocateUninitializedString(int length)
    {
        object obj = GC.AllocateUninitializedArray<char>(length);
        GetMethodTable(obj) = StringMT;
        return (string)obj;
    }

    public static void Main(string[] args)
    {
        string obj = AllocateUninitializedString(100);
        Console.WriteLine(obj.GetType());
    }
}