how can i store and reuse pieces of my lambda expressions

问题

I have a block of code where a piece of the lambda expression is used again and again. How can store this logic so that i can reuse this expression piece?

Eg: lets take the example of the code given below

Session.Query<DimensionGroup>()(dimgroup=>(dimgroup.Users.Where(map => 
((map.User.Key == _users.PublicUser.Key || map.User.Key == _users.CurrentUser.Key) &&
map.AccessLevel.ToAccessLevel() == AccessLevel.Write)).Count() > 0));

(map.User.Key == _users.PublicUser.Key || map.User.Key == _users.CurrentUser.Key) being the portion I want to reuse.

and a similar piece of code...

Session.Query<DimensionGroup>()(dimgroup =>(dimgroup.Users.Where(map => ((map.User.Key
==_users.PublicUser.Key || map.User.Key == _users.CurrentUser.Key) &&
map.AccessLevel.ToAccessLevel() ==  AccessLevel.Read)).Count() > 0));

(map.User.Key == _users.PublicUser.Key || map.User.Key == _users.CurrentUser.Key) being the portion I want to reuse.

Is there some way I can reuse just those parts of the expression?

回答1:

The easiest way is to reuse a single lamda expression, like so:

Expression<Func<User, bool>> KeysMatch = 
    map => map.User.Key == _users.PublicUser.Key 
        || map.User.Key == _users.CurrentUser.Key;

Session.Query<DimensionGroup>()(dimgroup=>(
    dimgroup.Users.Where(KeysMatch)
    .Where(map => map.AccessLevel.ToAccessLevel() == AccessLevel.Write))
    .Count() > 0
));

The next step up is to actually modify expression trees themselves by invoking lambda expressions. This is more complicated, and unless you want to get down and dirty with it is easier with a toolkit. I suggest LinqKit.

回答2:

You haven't told us if the lambda expressions are being converted to expression-trees or to delegates, but assuming the former, I recommend using PredicateBuilder.

Expression<Map, bool> predicate = map =>  map.User.Key == _users.PublicUser.Key 
                                       || map.User.Key == _users.CurrentUser.Key;

Then your first query becomes:

var pred1 = predicate.And
            (map => map.AccessLevel.ToAccessLevel() == AccessLevel.Write);

var query1 = Session.Query<DimensionGroup>
    (dimgroup => dimgroup.Users.Where(pred1).Count() > 0);

And your second:

var pred2 = predicate.And
            (map => map.AccessLevel.ToAccessLevel() == AccessLevel.Read);

var query2 = Session.Query<DimensionGroup>
    (dimgroup => dimgroup.Users.Where(pred2).Count() > 0);

As you can see, there is scope for factoring out even more commonalities; since the queries appear to only differ by different AccessLevels.

回答3:

Func<Map, bool> func = (map => ((((map.User.Key ==_users.PublicUser.Key || map.User.Key == _users.CurrentUser.Key) && map.AccessLevel.ToAccessLevel() == AccessLevel.Read)).Count() > 0));

EDIT:

you could try something like this.

class MyClass{

 Func<Map, bool> func = null;

 MyClass()
 {
    func = (map => ((((map.User.Key ==_users.PublicUser.Key || map.User.Key == _users.CurrentUser.Key) && map.AccessLevel.ToAccessLevel() == AccessLevel.Read)).Count() > 0));
  }
  ...
  ...
}

来源：https://stackoverflow.com/questions/5397027/how-can-i-store-and-reuse-pieces-of-my-lambda-expressions

标签

lambda

expression