if-else通过where子句c＃中的linq-to-sql查询等效

Question

Basically I wanted to have an if-else statement in my linq to sql statement.

var query = from d in database
            if(x == y) {
                where d.Attr = x
            }
            else {
                 where d.Attr = y
            }
            select d;

Any ideas?

Answer 1

Supposing, you meant == and not = :

from d in database
where (x == y && d.Attr == x) ||
      (x != y && d.Attr == y)
select d;

Answer 2

Isn't that just the same as

var query = from d in database
            where d.Attr == y
            select d;

Your situation (or a similar, more realistic one) can be handled with a simple conditional in the where clause. For more complex queries you can use the extension methods to build up a query implementing conditionals pretty easily or use a PredicateBuilder to create arbitrarily complex conditions.

var query = db.Table;
if (x == y)
{
   query = query.Where( d.Attr == z );
}
else
{
   query = query.Where( d.Attr == t );
}

var predicate = PredicateBuilder.True<Foo>()
                                .And( f => f.Attr == y )
                                .And( f => f.Attr == x )
                                .Or( f => f.Attr == z );
var query = db.Foo.Where( predicate );

Answer 3

EDIT: I think I may have misunderstood what you wanted to do.

I'm not sure if the ? (ternary) operator will work in Linq to SQL, but try this:

from d in database select (x == y) ? x : y

Answer 4

wouldnt

if(x == y) { 
                where d.Attr = x 
            } 

be the same as

if(x == y) { 
                where d.Attr = y 
            } 

if x==y?

so couldnt you just use

where d.Attr = y

Answer 5

This is the solution, assuming you meant == and not = :

var query = from d in database
            where (x == y ? d.Attr == x : d.Attr == y)
            select d;

However, this is logically equal to the following:

var query = from d in database
            where d.Attr == y
            select d;

Because, if x == y is true, then d.Attr == x and d.Attr == y will be equal.

Answer 6

var query = from d in database where (x == y)? d.Attr = x : d.Attr = y select d;