AWS SDK Version 3 for .NET
API Reference

AWS services or capabilities described in AWS Documentation may vary by region/location. Click Getting Started with Amazon AWS to see specific differences applicable to the China (Beijing) Region.

This operation initiates a multipart upload and returns an upload ID. This upload ID is used to associate all of the parts in the specific multipart upload. You specify this upload ID in each of your subsequent upload part requests (see UploadPart). You also include this upload ID in the final request to either complete or abort the multipart upload request.

For more information about multipart uploads, see Multipart Upload Overview.

If you have configured a lifecycle rule to abort incomplete multipart uploads, the upload must complete within the number of days specified in the bucket lifecycle configuration. Otherwise, the incomplete multipart upload becomes eligible for an abort operation and Amazon S3 aborts the multipart upload. For more information, see Aborting Incomplete Multipart Uploads Using a Bucket Lifecycle Policy.

For information about the permissions required to use the multipart upload API, see Multipart Upload API and Permissions.

For request signing, multipart upload is just a series of regular requests. You initiate a multipart upload, send one or more requests to upload parts, and then complete the multipart upload process. You sign each request individually. There is nothing special about signing multipart upload requests. For more information about signing, see Authenticating Requests (AWS Signature Version 4).

After you initiate a multipart upload and upload one or more parts, to stop being charged for storing the uploaded parts, you must either complete or abort the multipart upload. Amazon S3 frees up the space used to store the parts and stop charging you for storing them only after you either complete or abort a multipart upload.

You can optionally request server-side encryption. For server-side encryption, Amazon S3 encrypts your data as it writes it to disks in its data centers and decrypts it when you access it. You can provide your own encryption key, or use AWS Key Management Service (AWS KMS) customer master keys (CMKs) or Amazon S3-managed encryption keys. If you choose to provide your own encryption key, the request headers you provide in UploadPart) and UploadPartCopy) requests must match the headers you used in the request to initiate the upload by using CreateMultipartUpload.

To perform a multipart upload with encryption using an AWS KMS CMK, the requester must have permission to the kms:Encrypt, kms:Decrypt, kms:ReEncrypt*, kms:GenerateDataKey*, and kms:DescribeKey actions on the key. These permissions are required because Amazon S3 must decrypt and read data from the encrypted file parts before it completes the multipart upload.

If your AWS Identity and Access Management (IAM) user or role is in the same AWS account as the AWS KMS CMK, then you must have these permissions on the key policy. If your IAM user or role belongs to a different account than the key, then you must have the permissions on both the key policy and your IAM user or role.

For more information, see Protecting Data Using Server-Side Encryption.

Access Permissions

When copying an object, you can optionally specify the accounts or groups that should be granted specific permissions on the new object. There are two ways to grant the permissions using the request headers:

  • Specify a canned ACL with the x-amz-acl request header. For more information, see Canned ACL.

  • Specify access permissions explicitly with the x-amz-grant-read, x-amz-grant-read-acp, x-amz-grant-write-acp, and x-amz-grant-full-control headers. These parameters map to the set of permissions that Amazon S3 supports in an ACL. For more information, see Access Control List (ACL) Overview.

You can use either a canned ACL or specify access permissions explicitly. You cannot do both.

Server-Side- Encryption-Specific Request Headers

You can optionally tell Amazon S3 to encrypt data at rest using server-side encryption. Server-side encryption is for data encryption at rest. Amazon S3 encrypts your data as it writes it to disks in its data centers and decrypts it when you access it. The option you use depends on whether you want to use AWS managed encryption keys or provide your own encryption key.

  • Use encryption keys managed by Amazon S3 or customer master keys (CMKs) stored in AWS Key Management Service (AWS KMS) – If you want AWS to manage the keys used to encrypt data, specify the following headers in the request.

    • x-amz-server-side​-encryption

    • x-amz-server-side-encryption-aws-kms-key-id

    • x-amz-server-side-encryption-context

    If you specify x-amz-server-side-encryption:aws:kms, but don't provide x-amz-server-side-encryption-aws-kms-key-id, Amazon S3 uses the AWS managed CMK in AWS KMS to protect the data.

    All GET and PUT requests for an object protected by AWS KMS fail if you don't make them with SSL or by using SigV4.

    For more information about server-side encryption with CMKs stored in AWS KMS (SSE-KMS), see Protecting Data Using Server-Side Encryption with CMKs stored in AWS KMS.

  • Use customer-provided encryption keys – If you want to manage your own encryption keys, provide all the following headers in the request.

    • x-amz-server-side​-encryption​-customer-algorithm

    • x-amz-server-side​-encryption​-customer-key

    • x-amz-server-side​-encryption​-customer-key-MD5

    For more information about server-side encryption with CMKs stored in AWS KMS (SSE-KMS), see Protecting Data Using Server-Side Encryption with CMKs stored in AWS KMS.

Access-Control-List (ACL)-Specific Request Headers

You also can use the following access control–related headers with this operation. By default, all objects are private. Only the owner has full access control. When adding a new object, you can grant permissions to individual AWS accounts or to predefined groups defined by Amazon S3. These permissions are then added to the access control list (ACL) on the object. For more information, see Using ACLs. With this operation, you can grant access permissions using one of the following two methods:

  • Specify a canned ACL (x-amz-acl) — Amazon S3 supports a set of predefined ACLs, known as canned ACLs. Each canned ACL has a predefined set of grantees and permissions. For more information, see Canned ACL.

  • Specify access permissions explicitly — To explicitly grant access permissions to specific AWS accounts or groups, use the following headers. Each header maps to specific permissions that Amazon S3 supports in an ACL. For more information, see Access Control List (ACL) Overview. In the header, you specify a list of grantees who get the specific permission. To grant permissions explicitly, use:

    • x-amz-grant-read

    • x-amz-grant-write

    • x-amz-grant-read-acp

    • x-amz-grant-write-acp

    • x-amz-grant-full-control

    You specify each grantee as a type=value pair, where the type is one of the following:

    • id – if the value specified is the canonical user ID of an AWS account

    • uri – if you are granting permissions to a predefined group

    • emailAddress – if the value specified is the email address of an AWS account

      Using email addresses to specify a grantee is only supported in the following AWS Regions:

      • US East (N. Virginia)

      • US West (N. California)

      • US West (Oregon)

      • Asia Pacific (Singapore)

      • Asia Pacific (Sydney)

      • Asia Pacific (Tokyo)

      • Europe (Ireland)

      • South America (São Paulo)

      For a list of all the Amazon S3 supported Regions and endpoints, see Regions and Endpoints in the AWS General Reference.

    For example, the following x-amz-grant-read header grants the AWS accounts identified by account IDs permissions to read object data and its metadata:

    x-amz-grant-read: id="11112222333", id="444455556666"

The following operations are related to CreateMultipartUpload:


This is an asynchronous operation using the standard naming convention for .NET 4.5 or higher. For .NET 3.5 the operation is implemented as a pair of methods using the standard naming convention of BeginInitiateMultipartUpload and EndInitiateMultipartUpload. For Unity the operation does not take CancellationToken as a parameter, and instead takes AmazonServiceCallback<InitiateMultipartUploadRequest, InitiateMultipartUploadResponse> and AsyncOptions as additional parameters.

Namespace: Amazon.S3
Assembly: AWSSDK.S3.dll
Version: 3.x.y.z


public abstract Task<InitiateMultipartUploadResponse> InitiateMultipartUploadAsync(
         String bucketName,
         String key,
         CancellationToken cancellationToken
Type: System.String

The name of the bucket to which to initiate the upload

Type: System.String

Object key for which the multipart upload is to be initiated.

Type: System.Threading.CancellationToken

A cancellation token that can be used by other objects or threads to receive notice of cancellation.

Return Value
The response from the InitiateMultipartUpload service method, as returned by S3.

Version Information

.NET Standard:
Supported in: 2.0, 1.3

.NET Framework:
Supported in: 4.5

Portable Class Library:
Supported in: Windows Store Apps
Supported in: Xamarin Android
Supported in: Xamarin iOS (Unified)
Supported in: Xamarin.Forms

Supported Versions: 4.6 and above
Supported Platforms: Android, iOS, Standalone

See Also