Use AmazonS3(.NET/C#)

UseAmazonS3.cs

using System.IO;
using System.Threading.Tasks;
using Amazon;
using Amazon.S3;
using Amazon.S3.Model;

namespace UseAmazonS3
{
    class Program
    {
        static async Task Main(string[] args)
        {
            var client = new AmazonS3Client(
                "*AccessKey*", 
                "*SecretAccessKey*",
                new AmazonS3Config
                {
                    RegionEndpoint = RegionEndpoint.APNortheast1
                });

            // Upload
            var response = await client.GetObjectAsync(new GetObjectRequest
            {
                BucketName = "*BucketName*",
                Key = "*KeyName*"
            });

            var stream = new MemoryStream();
            response.ResponseStream.CopyTo(stream);
            File.WriteAllBytes(@"c:\temp\*Keyname*", stream.ToArray());

            // Download
            await client.PutObjectAsync(new PutObjectRequest
            {
                BucketName = "*BucketName*",
                Key = "*KeyName*",
                InputStream = new MemoryStream(File.ReadAllBytes(@"c:\temp\*KeyName*"))
            });
        }
    }
}