如何用C#实现数据库的动态连接字符串？根据用户切换？

{
  "ConnectionStrings": {
    "Default": "Server=localhost;Database=CommonDB;..."
  },
  "UserDatabaseMapping": {
    "user1": "Server=localhost;Database=UserDB1;...",
    "user2": "Server=localhost;Database=UserDB2;...",
    "admin": "Server=localhost;Database=AdminDB;..."
  }
}

public class ConnectionStringService
{
    private readonly IConfiguration _configuration;

    public ConnectionStringService(IConfiguration configuration)
    {
        _configuration = configuration;
    }

    public string GetConnectionString(string userName)
    {
        var mapping = _configuration.GetSection("UserDatabaseMapping").Get<Dictionary<string, string>>();

        if (mapping != null && mapping.TryGetValue(userName.ToLower(), out string connString))
        {
            return connString;
        }

        // 默认使用公共库或抛出异常
        return _configuration.GetConnectionString("Default");
    }
}

public UserDbContextFactory(ConnectionStringService connService)
{
    _connService = connService;
}

public MyDbContext CreateContext(string userName)
{
    var connectionString = _connService.GetConnectionString(userName);
    var optionsBuilder = new DbContextOptionsBuilder<MyDbContext>();
    optionsBuilder.UseSqlServer(connectionString);
    return new MyDbContext(optionsBuilder.Options);
}

<p>在控制器中使用：</p>
```csharp
[ApiController]
[Route("[controller]")]
public class DataController : ControllerBase
{
    private readonly UserDbContextFactory _dbContextFactory;

    public DataController(UserDbContextFactory dbContextFactory)
    {
        _dbContextFactory = dbContextFactory;
    }

    [HttpGet]
    public IActionResult GetData([FromHeader]string username)
    {
        using var context = _dbContextFactory.CreateContext(username);
        var data = context.Data.ToList();
        return Ok(data);
    }
}

基本上就这些。关键点是把连接字符串的决策逻辑封装好，结合用户上下文动态提供，同时注意安全（不要拼接敏感信息）、性能（缓存）和可维护性。