如何让Linq2Sql了解自定义类型？(How to make the Linq2Sql understand custom types?)

 乡亲！ 
 
 假设我们有一组接口提出问题'域： IUser, IAddressBook, IComment等。 假设IUser定义如下： 
 
public interface IUser : IAmIdentifiedEntity<int>, IHaveName
{
    string FullName { get; set; }
    string Email { get; set; }
    bool ReceiveNotification { get; set; }
}

public interface IHaveName
{
    string Name { get; set; }
}
 
 在我的应用程序中，我只使用提到的合同，例如： 
 
public IUser GetUser(string userName)
{
    return Warehouse.GetRepository<IUser>().GetAll()
        .First(u => u.Name == userName);
}
 
 如您所见，我正在使用一些网关来获取数据。 Repository'方法GetAll()返回IQueryable<TEntity> ，因此可以构建一些复杂的查询并使用延迟加载的所有好处。 当我介绍它的时候，我想到了Linq2Sql的设备。 
 
 有一段时间，在开发客户端代码时，我们使用“内存中”实现数据存储。 所以一切正常。 但现在是将域绑定到SQL Server的时候了。 所以我已经开始在Linq2Sql上实现所有的基础设施......当得到简单的解决方案（受到Fredrik Kalseth 文章的启发）并得到第一个例外时，我已经意识到这个想法的所有悲伤......这是IUser实施： 
 
public partial class USER : IUser
{
    int IHaveID<int>.ID
    {
        get { return USERID; }
    }

    string IHaveName.Name
    {
        get { return USERNAME;  }
        set { USERNAME = value; }
    }

    string IUser.FullName 
    {
        get { return USERFULLNAME; }
        set { USERFULLNAME = value; }
    }

    // ... same approach for other properties
}
 
 在这里 - 例外： 
 
Exception: System.NotSupportedException: 
    The member 'Data.IHaveName.Name' has no supported translation to SQL.
 
 这是明显的例外 - Linq2Sql提供程序不了解外部接口，但我该如何解决呢？ 我无法更改域接口以返回Linq.Expression，如： 
 
Expression<Func<string>> IHaveName.Name
{
    get { return (() => USERNAME);  }
    set { USERNAME = value(); }
}
 
 因为它打破了域接口的所有当前代码使用，而且由于宗教信仰，我不能用Expression<Func<int>>替换int :) 
 
 也许，我应该写自定义Expression访问者跳过查询'AST调用IUser.SomeProperty并用它的内部子AST替换它... 
 
 你能提出你的想法吗？ 
 
 UPDATE。 在这里，我应该写一些关于Repository实现的东西。 看看GetAll()源代码： 
 
public IQueryable<TEntity> GetAll()
{
    ITable table = GetTable();
    return table.Cast<TEntity>();
}

protected ITable GetTable()
{
    return _dataContext.GetTable(_implType);
}
 
 在我的示例中， TEntity <=> IUser和_implType <=> typeof(USER) 
 
 更新2.我发现了相关的问题 ，OP有一个类似的问题：需要在具体的ORM实体和它的域实体之间建立一些桥梁。 我发现有趣的答案 ：作者建议创建表达式访问者，它将执行实体之间的转换（ORM <=>域）。 

folks!
 
Suppose we have set of interfaces that presents problem' domain: IUser, IAddressBook, IComment and so on. The assume that IUser is defined as follows:
 
public interface IUser : IAmIdentifiedEntity<int>, IHaveName
{
    string FullName { get; set; }
    string Email { get; set; }
    bool ReceiveNotification { get; set; }
}

public interface IHaveName
{
    string Name { get; set; }
}
 
In my application I use only mentioned contracts, for example:
 
public IUser GetUser(string userName)
{
    return Warehouse.GetRepository<IUser>().GetAll()
        .First(u => u.Name == userName);
}
 
As you can see, I'm using some gateway to get data. Repository' method GetAll() returns IQueryable<TEntity>, so it is possible to build some complex query and use all benefits of lazy loading. When I was introducing it, I thought about appliance of Linq2Sql in future.
 
For some time, while developing client code, we were using 'in-memory' implementation of data storage. So everything works fine. But now it's a time to bind domain to SQL Server. So I've started to implement all the infrastructure upon Linq2Sql... And when made simple solution (inspired by article of Fredrik Kalseth) and got first exception, I've realized all sadness of that idea... Here is implmentation of IUser:
 
public partial class USER : IUser
{
    int IHaveID<int>.ID
    {
        get { return USERID; }
    }

    string IHaveName.Name
    {
        get { return USERNAME;  }
        set { USERNAME = value; }
    }

    string IUser.FullName 
    {
        get { return USERFULLNAME; }
        set { USERFULLNAME = value; }
    }

    // ... same approach for other properties
}
 
And here - exception:
 
Exception: System.NotSupportedException: 
    The member 'Data.IHaveName.Name' has no supported translation to SQL.
 
This is obvious exception - Linq2Sql provider doesn't understand external interfaces, but how should I resolve it? I can't change domain interfaces to return Linq.Expression like:
 
Expression<Func<string>> IHaveName.Name
{
    get { return (() => USERNAME);  }
    set { USERNAME = value(); }
}
 
because it breaks all the current code usage of domain interfaces and moreover i can't replace int with Expression<Func<int>> because of religious beliefs :)
 
Maybe, I should write custom Expression visitor to skip in query' AST calling of IUser.SomeProperty and replace it with it's inner sub-AST...
 
Can you provide your thoughts on it?
 
UPDATE. Here I should write something about Repository implementation. Look at GetAll() source:
 
public IQueryable<TEntity> GetAll()
{
    ITable table = GetTable();
    return table.Cast<TEntity>();
}

protected ITable GetTable()
{
    return _dataContext.GetTable(_implType);
}
 
In my example, TEntity <=> IUser and _implType <=> typeof(USER)
 
UPDATE 2. I've found related question, where OP has an alike problem: need some bridge between concrete ORM entities and it's domain entities. And I've found interesting the answer: author suggested to create expression visitor, which will perform convertion between entities (ORM <=> Domain).

原文：https://stackoverflow.com/questions/7901617