public class CalculatingSha256
{
    private static object lockObj = new object();
    private static SortedList sha256DataList;

    public CalculatingSha256()
    {
        sha256DataList = new SortedList();
        sha256DataList = SortedList.Synchronized(sha256DataList);
    }

    public void CalculateSha256(PartBlock block)
    {
        int index = sha256DataList.IndexOfKey(block.IdFullBlock);
        SHA256Data sha256Data = null;

        if (index >= 0)
        {
            sha256Data = (SHA256Data)sha256DataList.GetByIndex(index);
        }

        if (sha256Data == null)
        {
            sha256Data = new SHA256Data();
            if (block.SizeFullBlock != 0)
            {
                sha256Data.BlockSize = block.SizeFullBlock;
            }
            sha256Data.Sha256 = new SHA256Managed();
            lock (lockObj)
            {
                sha256DataList.Add(block.IdFullBlock, sha256Data);
            }
        }

        Console.WriteLine(block.Id+"  "+block.SizeFullBlock+"  "+block.IdFullBlock);
        sha256Data.Sha256.TransformBlock(block.Data, 0, block.Size, block.Data, 0);
        sha256Data.CurrentCalculated += block.Size;
        bool bBlockCalculateIsOver = sha256Data.CurrentCalculated == sha256Data.BlockSize;

        if ((sha256Data.BlockSize != 0) && (bBlockCalculateIsOver))
        {
            byte[] input = new byte[0];
            sha256Data.Sha256.TransformFinalBlock(input, 0, 0);
            string hashString = ToString(sha256Data.Sha256.Hash);
            Console.WriteLine("Блок номер {0}:{1}", block.IdFullBlock+1, hashString.ToUpper());
            sha256DataList.Remove(block.IdFullBlock);
        }
    }
}

public abstract class Block
{
    public int Id { get; set; }
}

public class PartBlock : Block
{
    public int Size { get; set; }
    public byte[] Data { get; set; }
    //номер целого блока которому принадлежит
    public int IdFullBlock { get; set; }
    public int SizeFullBlock { get; set; }
}

public class SHA256Data
{
    public int BlockSize { get; set; }
    public int CurrentCalculated { get; set; }
    public SHA256Managed Sha256 { get; set; }
}

byte[] hash;
using (var fileStream = File.Open(path))
    hash = SHA256Managed.Create().ComputeHash(fileStream);