如何使用DDD和.NET上传和下载附件？

在DDD架构中，将业务逻辑封装在领域层中，通过应用层和基础层进行交互。为了实现上传和下载附件的功能，我们可以在领域层中定义一个附件实体，并在基础层中实现相应的存储和读取方法。

附件实体示例：

public class Attachment { public Guid Id { get; set; } public string FileName { get; set; } public byte[] Content { get; set; } }

存储方法示例：

public class AttachmentRepository : IAttachmentRepository { private readonly string _connectionString;

public AttachmentRepository(string connectionString)
{
    _connectionString = connectionString;
}

public async Task<Attachment> SaveAsync(Attachment attachment)
{
    using (var connection = new SqlConnection(_connectionString))
    {
        await connection.OpenAsync();

        var sql = @"
            INSERT INTO [Attachments] ([Id], [FileName], [Content])
            VALUES (@Id, @FileName, @Content)";

        await connection.ExecuteAsync(sql, new
        {
            attachment.Id,
            attachment.FileName,
            attachment.Content
        });

        return attachment;
    }
}

}

读取方法示例：

public class AttachmentRepository : IAttachmentRepository { private readonly string _connectionString;

public AttachmentRepository(string connectionString)
{
    _connectionString = connectionString;
}

public async Task<Attachment> GetByIdAsync(Guid id)
{
    using (var connection = new SqlConnection(_connectionString))
    {
        await connection.OpenAsync();

        var sql = @"
            SELECT [Id], [FileName], [Content]
            FROM [Attachments]
            WHERE [Id] = @Id";

        return await connection.QueryFirstOrDefaultAsync<Attachment>(sql, new { id });
    }
}

}

在应用层中实现上传和下载附件的业务逻辑：

public class AttachmentService : IAttachmentService { private readonly IAttachmentRepository _attachmentRepository;

public AttachmentService(IAttachmentRepository attachmentRepository)
{
    _attachmentRepository = attachmentRepository;
}

public async Task<Attachment> SaveAsync(string fileName, byte[] content)
{
    var attachment = new Attachment
    {
        Id = Guid.NewGuid(),
        FileName = fileName,
        Content = content
    };

    return await _attachmentRepository.SaveAsync(attachment);
}

public async Task<byte[]> GetByIdAsync(Guid id)
{
    var attachment = await _attachmentRepository.GetByIdAsync(id);

    if (attachment == null)
    {
        throw new ArgumentException("Attachment not found.");
    }

    return attachment.Content;
}

}

以上