How to set Http Header for Amazon S3 programmatically?

For AWSSDK.S3 V3.* its works like this:

request.Headers.Expires = DateTime.Now.AddMinutes(2);

If you are using the AWS SDK 2.X then the "AddHeader" method is no longer available. To add a header you just modify the header collection directly.

req.Headers["expires"] = "Thu, 01 Dec 1994 16:00:00 GMT";

Here is the modified example Geoff used above:

var client = new Amazon.S3.AmazonS3Client(AWS_Key, AWS_SecretKey);

var req = PutObjectRequest req= new PutObjectRequest()
{
    BucketName = "mybucket",
    Key = "myfile.txt",
    FilePath = @"C:\myfile.txt"
};

req.Headers["expires"] = "Thu, 01 Dec 1994 16:00:00 GMT";

client.PutObject(req);

To change the header it's the same way:

var req = new Amazon.S3.Model.CopyObjectRequest()
{
    MetadataDirective = S3MetadataDirective.REPLACE,
    SourceBucket = "mybucket",
    SourceKey = "myfile.txt",
    DestinationBucket = "mybucket",
    DestinationKey = "myfile.txt"
};

req.Headers["expires"] = "Thu, 01 Dec 1994 16:00:00 GMT";

client.CopyObject(req);

As you are using Asp.net, I assume you are using the AWS .NET SDK.

To add the Expires(or any other http header) when uploading the object, add it as part of the PutObject request.

var client = new Amazon.S3.AmazonS3Client(AWS_Key, AWS_SecretKey);

var req = new Amazon.S3.Model.PutObjectRequest()
                 .WithFilePath(@"C:\myfile.txt")
                 .WithKey("myfile.txt")
                 .WithBucketName("mybucket");

req.AddHeader("expires", "Thu, 01 Dec 1994 16:00:00 GMT");

client.PutObject(req);

To change the header on an existing object, you need to copy the object to itself.

var req = new Amazon.S3.Model.CopyObjectRequest()
                 .WithDirective(Amazon.S3.Model.S3MetadataDirective.REPLACE)
                 .WithSourceBucket("mybucket")
                 .WithSourceKey("myfile.txt")
                 .WithDestinationBucket("mybucket")
                 .WithDestinationKey("myfile.txt");

req.AddHeader("expires", "Thu, 01 Dec 1994 16:00:00 GMT");

client.CopyObject(req);

Note: .WithDirective(Amazon.S3.Model.S3MetadataDirective.REPLACE) must be set in order to specify new headers. Otherwise the existing headers are just copied over.

More more info see the .NET SDK docs.