Dynamic where clause in LINQ - with column names a

问题:

Disclaimer: I've solved the problem using Expressions from System.Linq.Expressions, but I'm still looking for a better/easier way.

Consider the following situation :

var query = 
    from c in db.Customers
    where (c.ContactFirstName.Contains("BlackListed") || 
           c.ContactLastName.Contains("BlackListed")  ||
           c.Address.Contains("BlackListed"))
    select c;

The columns/attributes that need to be checked against the blacklisted term are only available to me at runtime. How do I generate this dynamic where clause?

An additional complication is that the Queryable collection (db.Customers above) is typed to a Queryable of the base class of 'Customer' (say 'Person'), and therefore writing c.Address as above is not an option.

回答1:

@Geoff has the best option, justing Dynamic LINQ.

If you want to go the way of building queries at runtime using Lambda though I'd recomment that you use the PredicateBuilder (http://www.albahari.com/nutshell/predicatebuilder.aspx) and have something such as this:

Expression<Fun<T,bool>> pred = null; //delcare the predicate to start with. Note - I don't know your type so I just used T 
if(blacklistFirstName){
  pred = p => p.ContactFirstName.Contains("Blacklisted");
}
if(blacklistLastName){
  if(pred == null){
    pred = p => p.ContactLastName.Contains("Blacklisted"); //if it doesn't exist just assign it
  }else{
    pred = pred.And(p => p.ContactLastName.Contains("Blacklisted"); //otherwise we add it as an And clause
  }
}

And so on for all the columns you want to include. When you get to your query you just need something like this:

var results = db.Customers.Where(pred).Select(c => c);

I've used this to do building of LINQ for searching where there are about 20 different options and it produces really good SQL.

回答2:

var query = from C in db.Customers select c;

if (seachFirstName)
         query = query.Where(c=>c.ContactFirstname.Contains("Blacklisted"));

if (seachLastName)
         query = query.Where(c=>c.ContactLastname.Contains("Blacklisted"));

if (seachAddress)
         query = query.Where(c=>c.Address.Contains("Blacklisted"));

Note that they aren't mutually exclusive.

回答3:

You can turn your where clauses on and off using some logic expressions.

//Turn on all where clauses
bool ignoreFirstName = false;
bool ignoreLastName = false;;
bool ignoreAddress = false;

//Decide which WHERE clauses we are going to turn off because of something.
if(something)
    ignoreFirstName = true; 

//Create the query
var queryCustomers = from c in db.Customers 
    where (ignoreFirstName || (c.ContactFirstName.Contains("BlackListed")))
    where (ignoreLastName || (c.ContactLastName.Contains("BlackListed")))
    where (ignoreAddress || (c.Address.Contains("BlackListed"))
    select j;  

If ignoreFirstName is true in the query then the condition on the other side of the or statement will be ignored.

回答4:

Since this is not LINQ to Objects, but rather LINQ to SQL, you have no other alternative beside using either Expressions or a stored procedure.