C# byte[] to List<byte[]>

Your byte array is a reference type, which means changing it in each loop changes the data stored. Declaring it inside of each loop should work:

 byte[] a = {1,2,3,4,4,2,3,4,2,5,3,4,4,2,6,3,4,5,3,3};
 List<byte[]> b = new List<byte[]>();

 for (int u=0; u<4; u++)
 {
     byte[] inter_byte= new byte[5];
     for (int p=0; p<5; p++)
     {
         inter_byte[p] = a[(5*u) + p];
     }
     b.Add(inter_byte);
 }        

You need to reallocate inter_byte in each iteration, otherwise it's getting reused and you're replacing the rows.

something like this should do it... (unless i misunderstood the question)

        List<byte[]> b = a.Select((by, i) => new { group = i / 5, value = by })
            .GroupBy(item => item.group)
            .Select(group => group.Select(v => v.value).ToArray())
            .ToList();

groups the bytes into arrays of 5 into a list.

inter_byte is a reference to an array of bytes. You are only allocating the actual array of bytes once (with the new byte[5]. You need to do that in your loop.

Here's a nice extension method for what you want to do, but it's a bit safer because it won't run into out of range issues.

    public static IList<T[]> GroupArray<T>(this T[] array, int groupSize)
    {
        if (array == null)
            throw new ArgumentNullException("array");
        if (groupSize <= 0)
            throw new ArgumentException("Group size must be greater than 0.", "groupSize");

        IList<T[]> list = new List<T[]>();

        T[] temp = new T[groupSize];

        for (int i = 0; i < array.Length; i++)
        {
            if ((i % groupSize) == 0)
            {
                temp = new T[groupSize];
                list.Add(temp);
            }

            temp[(i % groupSize)] = array[i];
        }

        return list;
    }

SAMPLE USAGE:

        Byte[] myByte = { 1, 2, 3, 4, 4, 2, 3, 4, 2, 5, 3, 4, 4, 2, 6, 3, 4, 5, 3, 3 };

        IList<Byte[]> myList = myByte.GroupArray(5);

        foreach (var item in myList)
        {
            Console.Write(item + " ");
            foreach (var item2 in item)
            {
                Console.Write(item2);
            }
            Console.WriteLine();
        }

ttry this :

byte[] a = {1,2,3,4,4,2,3,4,2,5,3,4,4,2,6,3,4,5,3,3};
List<byte[] b = new List<byte[]>();


for (int u=0; u<4; u++)
{
    byte[] inter_byte= new byte[5];
    for (int p=0; p<5; p++)
    {
         inter_byte[u] = file[(5*u) + p];
    }
    b.Add(inter_byte);
}

var a = new byte[]
            {
                1, 2, 3, 4, 4,
                2, 3, 4, 2, 5,
                3, 4, 4, 2, 6,
                3, 4, 5, 3, 3
            };

var b = new List<byte[]>();

int groupSize = 5;
for (int i = 0; i < a.Length; i += groupSize)
{
    int interSize = Math.Min(a.Length - i, groupSize);
    var interByte = new byte[interSize];

    Buffer.BlockCopy(a, i, interByte, 0, interSize);
    b.Add(interByte);
}

byte[] a = {1,2,3,4,4,2,3,4,2,5,3,4,4,2,6,3,4,5,3,3};
List<byte[]> b = new List<byte[]>();

for (int u=0; u<a.Count; u+=5)
{
     b.Add(a.Skip(u).Take(5).ToArray());
}