C# How to read a binary file into int[] without a BinaryReader loop?

You can use MemoryMarshal.AsBytes to read all data:

using var stream = new FileStream(...);
var target = new int[stream.Length / 4];
stream.Read(MemoryMarshal.AsBytes(target.AsSpan()));

No BinaryReader is used in that case. Be aware of endianness of int representation. This code above might cause problems if the file doesn't match to your hardware.

If you want to read the array as another type, you can use MemoryMarshal.Cast.

using System;
using System.Runtime.InteropServices;

class Program
{
    public static void Main(string[] args)
    {
        byte[] arrayByte = { 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88 };
        Span<int> spanInt = MemoryMarshal.Cast<byte, int>(arrayByte);

        Console.WriteLine("0x{0:X8}", spanInt[0]); // For little endian it will be 0x44332211.
        Console.WriteLine("0x{0:X8}", spanInt[1]); // For little endian it will be 0x88776655.
    }
}

Another alternative is Unsafe.As. However, there are some problems, such as Length not reflecting the converted type value. I recommend using the MemoryMarshal class.

using System;
using System.Runtime.CompilerServices;

class Program
{
    public static void Main(string[] args)
    {
        byte[] arrayByte = { 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88 };
        int[] arrayInt = Unsafe.As<byte[], int[]>(ref arrayByte);

        Console.WriteLine("Length={0}", arrayInt.Length); // 8!? omg...
        Console.WriteLine("0x{0:X8}", arrayInt[0]); // For little endian it will be 0x44332211.
        Console.WriteLine("0x{0:X8}", arrayInt[1]); // For little endian it will be 0x88776655.
    }
}