test.cs

// checks, if the blockchain is consistent by
// re-generating and comparing the hashes in each block
public bool IsValid(string blockHash = "") {
    // check all blocks
    int iNumberBlocks = Chain.Count;

    // check all blocks until the given, optional hash
    if (blockHash != "") {
        Block block = Chain.FirstOrDefault(h => h.BlockHash == blockHash);
        if (block != null)        {
            iNumberBlocks = block.Index + 1;
        }
    }

    // loop through all blocks, generate their hashes and compare
    for (int i = 1; i < iNumberBlocks; i++) {
        Block currentBlock = Chain[i];
        Block previousBlock = Chain[i - 1];

        if (currentBlock.BlockHash != currentBlock.GenerateBlockHash())
            return false;

        if (currentBlock.PreviousBlockHash != previousBlock.BlockHash)
            return false;
    }

    return true;
}