使用switch语句过滤AC＃LINQ语句

Question

I will use a simple example here to explain my problem. What I am trying to so is return a view model based upon a filtered LINQ query via a switch statement. My code reads like this:

var query = (
    from b in _db.Books
    join a in _db.Authors on b.AuthorId = a.Id
    select new BookViewModel
    {
       AuthorName = a.Name,
       BookName = b.Name
    });

switch (currentUser.Role)
{
     case "Admin": return query.ToList(); // Return all books for an admin.
     case "Publisher": return query.Where(x => x.Publisher == currentUser.PublisherId).ToList();
     default: throw new UnauthorizedAccessException(); // Given role is not authorized.
} 

In the "Publisher" case statement I want to filter and return the query by the current user's publisher ID, for example. But I cannot achieve this with my current code setup because this property does not exist in the BookViewModel object which I am selecting with my LINQ query.

I like this way of filtering using a switch statement. I find the code very readable. What is the most elegant way to achieve this please without my having to add the additional property to the view model?

Thanks.

Answer 1

If I understand correctly, you can use linq where with some logic to make it.

var query = (
    from b in _db.Books
    join a in _db.Authors on b.AuthorId = a.Id
    where (b.Publisher == currentUser.PublisherId && currentUser.Role == "Publisher") ||
          (currentUser.Role == "Admin")
    select new BookViewModel
    {
       AuthorName = a.Name,
       BookName = b.Name
    });

NOTE

The Publisher in the linq can change to your correctly context value.

Answer 2

The case statement case "Publisher": return query.Where(x => x.Publisher == currentUser.PublisherId).ToList(); is referring to a property in your BookViewModel when you say x.Publisher .

But you have not initialized that property with any value like you did with AuthorName and BookName , thus, Publisher will be null.

How to resolve this issue without adding the Publisher property to your view model depends entirely on how/where that property is stored, which is information you did not provide.

Answer 3

You can extract _db.Books as a subquery for easier management. I'm assuming Publisher is in your Books DbSet. If that's the case then:

var bookQueryable = _db.Books.AsQueryable();

switch (currentUser.Role)
{
     case "Admin": break;
     case "Publisher": bookQueryable = bookQueryable.Where(x => x.Publisher == currentUser.PublisherId);
     default: throw new UnauthorizedAccessException(); // Given role is not authorized.
} 

var query = (
from b in bookQueryable
join a in _db.Authors on b.AuthorId equals a.Id
select new BookViewModel
{
   AuthorName = a.Name,
   BookName = b.Name
});

return query.ToList();

Answer 4

The problem is that you want to use property Publisher from the result of your query, but you don't want to return Publisher as a property from your function.

This means, that when the Where is applied, Publisher should still be in your items, but after the Where it should have been removed.

The solution is: do the Select after the Where :

(I'm more accustomed to method syntax. Of course this method also works for query syntax

var initialQuery = db.Books
    .join(db.Authors                // join Books and Authors
    book => book.AuthorId,          // from every book take the AuthorId
    author => author.Id             // from every Author take the Id
    (book, author) => new           // when they match make one new object
    {                               // containing the matching book and author
        PublisherId = book.PublisherId,
        AuthorName = author.Name,
        BookTitle = book.Title,
    });

Note that the query is created, but it is not executed yet! You still can extend the query with other LINQ statements without much cost.

Luckily, your switch statement doesn't change the type of the initialQuery, so you can re-use it:

switch (switch (currentUser.Role)
{
    case "Admin": 
        // nothing to do
        break;
    case "Publisher":
        initialQuery =  initialQuery
            .Where(joinResult => joinResult.Book.Publisher == currentUser.PublisherId);
        break;
    default: 
        throw new UnauthorizedAccessException();
} 

Now do the select:

Problem!

Although you didn't say so, db.Books and db.Authors are tables in a database. They implement IQueryable<Book> and IQueryable<Author> . When executing the query you can only use statements that can be translated into SQL (or similar). This means that you can't use new BookViewModel while it is an IQueryable.

Solution: You'll have to make it AsEnumerable , after which you can do the final Select with your new BookViewModel

var queryResult = initialQuery
    // Transfer only the data you actually plan to use to your process
    .Select(joinResult => new
    { 
         AuthorName = joinResult.AuthorName,
         BookName = joinResult.BookTitle,
    })
    // move the selected data to local process in efficient way
    .AsEnumerable()
    // now you are free to use your local constructor:
    .Select(fetchedData => new BookViewModel()
    {
         BookName = fetchedData.BookName,
         AuthorName = fetchedData.AuthorName,
    });

By the way: still no query performed. Perform it now and return all fetched data:

return queryResult.ToList();

Answer 5

var query = (
    from b in _db.Books
    join a in _db.Authors on b.AuthorId = a.Id
    select new BookViewModel
    {
       AuthorName = a.Name,
       BookName = b.Name
    });

if (currentUser.Role.Equals("Admin")
{
    query = query.Where(x => x.Publisher == currentUser.PublisherId);
}

return query.ToList();

Authorization may be separated from logic by decorating the method with Authorize attribute (if you are using MVC/MVC Core).

Answer 6

When you do a join in LINQ, the compiler creates an anonymous object it refers to as a transparent identifier , which allows it to keep multiple range variables in scope. When you're building query expressions from parts, which can include a mix of LINQ expressions and extension method calls (as you'll see below), you can do this yourself, although I guess it would be an opaque identifier in that case.

I agree with @ErikE that your access checks should have already done before getting to this point, so the code that follows reflects that.

var query =
    from b in _db.Books
    join a in _db.Authors on b.AuthorId = a.Id
    select new { a, b };

// We'll test just the role that needs additional filtering.  Here,
// 'x' is the aforementioned opaque identifier.
if (currentUser.Role == "Publisher")
    query = query.Where(x => x.b.PublisherId == currentUser.PublisherId);

From here, you can build on it in further, like limiting the results to books that were illustrated by someone represented by a specific agent. This may be more than you need but it demonstrates the flexibility of the technique.

if (illustratorAgentId != null)
{
    // Join the Illustrators table and test its AgentId.  Since we only need the book
    // and author, we can continue to project 'x'.
    query =
        from x in query
        join ill in _db.Illustrators on x.b.IllustratorId equals ill.IllustratorId
        where ill.AgentId == illustratorAgentId
        select x;
}

And then when you're done building your query, you can project properties from your range variables (by way of your opaque identifier) as a BookViewModel .

IQueryable<BookViewModel> result =
    from x in query
    select new BookViewModel
    {
        AuthorName = x.a.Name,
        BookName = x.b.Name
    };

And then you can call any of the usual reification methods on result , which will probably be ToList in this case.