S3 Multipart ETag Calculation for C#. Refer : https://stackoverflow.com/questions/12186993/what-is-the-algorithm-to-compute-the-amazon-s3-etag-for-a-file-larger-than-5gb#answer-19896823

CalculateS3MutipartDigest.cs

void Main()
{
    var file = @"YOUR TARGET FILE";
    var bytes = FileHashes.GetFileBinary(file);
    var multipartETag = FileHashes.CalculateMultipartEtag(bytes, 16);
    multipartETag.Dump();
}

public static byte[] GetHash(this byte[] array, HashAlgorithm algorithm)
{
    var hash = algorithm.ComputeHash(array);
    return hash;
}

public static string CalculateMultipartEtag(byte[] array, int chunkCount)
{
    var multipartSplitCount = 0;
    var chunkSize = 1024 * 1024 * chunkCount;
    var splitCount = array.Length / chunkSize;
    var mod = array.Length - chunkSize * splitCount;
    IEnumerable<byte> concatHash = new byte[] {};

    using (var md5 = MD5.Create())
    {
        for (var i = 0; i < splitCount; i++)
        {
            var offset = i == 0 ? 0 : chunkSize * i;
            var chunk = GetSegment(array, offset, chunkSize);
            var hash = chunk.ToArray().GetHash(md5);
            concatHash = concatHash.Concat(hash);
            multipartSplitCount++;
        }
        if (mod != 0)
        {
            var chunk = GetSegment(array, chunkSize * splitCount, mod);
            var hash = chunk.ToArray().GetHash(md5);
            concatHash = concatHash.Concat(hash);
            multipartSplitCount++;
        }
        var multipartHash = concatHash.ToArray().GetHash(md5).ToHexString();
        return multipartHash + "-" + multipartSplitCount;
    }
}

private static ArraySegment<T> GetSegment<T>(this T[] array, int offset, int? count = null)
{
    if (count == null) { count = array.Length - offset; }
    return new ArraySegment<T>(array, offset, count.Value);
}

public static string ToHashString(this byte[] hash)
{
    return BitConverter.ToString(hash).Replace("-", "").ToLower();
}

public static string ToHexString(this byte[] bytes)
{
    var chars = new char[bytes.Length * 2 + 2];

    for (int i = 0; i < bytes.Length; i++)
    {
        chars[2 * i + 2] = ToHexDigit(bytes[i] / 16);
        chars[2 * i + 3] = ToHexDigit(bytes[i] % 16);
    }

    return new string(chars).ToLower();
}

private static char ToHexDigit(int i)
{
    if (i < 10)
    {
        return (char)(i + '0');
    }
    return (char)(i - 10 + 'A');
}