Adapter Pattern — Provider Abstraction in .NET

Definition

The Adapter pattern converts the interface of an existing class into the interface expected by the client, allowing incompatible components to collaborate. The adapter wraps the existing class and translates calls.

Diagram

classDiagram
    class IBlobStoreProvider {
        +SaveAsync()
        +OpenReadAsync()
        +DeleteAsync()
        +GetSizeAsync()
    }

    class AmazonS3Client {
        +PutObjectAsync()
        +GetObjectAsync()
        +DeleteObjectAsync()
    }

    class S3BlobClient {
        -s3Client : AmazonS3Client
    }

    IBlobStoreProvider <|.. S3BlobClient
    S3BlobClient --> AmazonS3Client : adapts

    class ISmtpTransport {
        +SendAsync()
    }

    class SmtpClient {
        +ConnectAsync()
        +SendAsync()
    }

    class MailKitSmtpTransport {
        -smtpClient : SmtpClient
    }

    ISmtpTransport <|.. MailKitSmtpTransport
    MailKitSmtpTransport --> SmtpClient : adapts

Implementation in Granit

Adapter	File	Target interface	Adapted class
`S3BlobClient`	`src/Granit.BlobStorage.S3/Internal/S3BlobClient.cs`	`IBlobStoreProvider` + `IPresignedUrlProvider`	`AmazonS3Client` (AWS SDK)
`MailKitSmtpTransport`	`src/Granit.Notifications.Smtp/Internal/MailKitSmtpTransport.cs`	`ISmtpTransport`	`SmtpClient` (MailKit, sealed)

Rationale

S3BlobClient isolates the framework from the AWS SDK, allowing provider changes (European hosting, MinIO) without touching the core. MailKitSmtpTransport wraps the sealed MailKit SmtpClient behind a testable ISmtpTransport interface.

Both ports are internal to their owning package: adapters are a framework extension point, not a host extension point. Hosts select a provider through its registration extension rather than by implementing the port.

Usage example

Swapping AWS S3 for MinIO is the same adapter pointed at a different endpoint — AddGranitBlobStorageS3() binds S3BlobOptions from the BlobStorage configuration section:

// appsettings.Development.json — credentials come from Granit.Vault in production.
{
  "BlobStorage": {
    "ServiceUrl": "http://localhost:9000",
    "ForcePathStyle": true
  }
}

builder.AddGranitBlobStorageS3();

// The rest of the application code remains unchanged
IBlobStorage blobStorage = serviceProvider.GetRequiredService<IBlobStorage>();
PresignedUploadTicket ticket = await blobStorage.InitiateUploadAsync(
    "medical-documents",
    new BlobUploadRequest("mri-report.pdf", "application/pdf", MaxAllowedBytes: 50_000_000),
    cancellationToken);