Linq-to-SQL：忽略WHERE子句中的空参数

Question

The query below should return records that either have a matching Id supplied in ownerGroupIds or that match ownerUserId . However is ownerUserId is null, I want this part of the query to be ignored.

    public static int NumberUnderReview(int? ownerUserId, List<int> ownerGroupIds)
    {
        return ( from c in db.Contacts
                 where 
                 c.Active == true 
                 &&
                 c.LastReviewedOn <= DateTime.Now.AddDays(-365) 
                 &&
                 ( // Owned by user
                    !ownerUserId.HasValue || 
                    c.OwnerUserId.Value == ownerUserId.Value
                 )
                 &&
                 ( // Owned by group
                    ownerGroupIds.Count == 0 ||
                    ownerGroupIds.Contains( c.OwnerGroupId.Value )
                 )
                 select c ).Count();
    }

However when a null is passed in for ownerUserId then I get the following error: Nullable object must have a value.

I get a tingling I may have to use a lambda expression in this instance?

Answer 1

your issue is that your are not passing in a nullable int, you are passing in a null.

try this:

Print(null);

private void Print(int? num)
{
     Console.WriteLine(num.Value);
}

and you get the same error.

It should work if you do this:

var q = ( from c in db.Contacts
                 where 
                 c.Active == true 
                 &&
                 c.LastReviewedOn <= DateTime.Now.AddDays(-365) 
                 &&
                 ( // Owned by group
                    ownerGroupIds.Count == 0 ||
                    ownerGroupIds.Contains( c.OwnerGroupId.Value )
                 )
                 select c );

if(ownerUserId != null && ownerUserId.HasValue)
     q = q.Where(p => p.OwnerUserId.Value == ownerUserId.Value);

return q.Count();

Answer 2

Have you some contacts with OwnerUserId null? If yes, c.OwnerUserId could be null and not having any value in c.OwnerUserId.Value

Answer 3

What about conditionally adding the where clause to the expression tree?

public static int NumberUnderReview(int? ownerUserId, List<int> ownerGroupIds)
    {

    var x = ( from c in db.Contacts
                 where 
                 c.Active == true 
                 &&
                 c.LastReviewedOn <= DateTime.Now.AddDays(-365) 
                 &&
                 ( // Owned by group
                    ownerGroupIds.Count == 0 ||
                    ownerGroupIds.Contains( c.OwnerGroupId.Value )
                 )
                 select c );

    if (ownerUserId.HasValue) {
        x = from a in x
            where c.OwnerUserId.Value == ownerUserId.Value
    }

    return x.Count();
    }

Answer 4

PROBLEM: "&&" and "||" is converted to a method like "AndCondition(a, b)", so "!a.HasValue || a.Value == b" becomes "OrCondition(!a.HasValue, a.Value == b);" The reason for this is probably to get a generic solution to work for both code and SQL statements. So instead, use the "?:" notation.

For more, see my blog post: http://peetbrits.wordpress.com/2008/10/18/linq-breaking-your-logic/

// New revised code.
public static int NumberUnderReview(int? ownerUserId, List<int> ownerGroupIds)
{
    return ( from c in db.Contacts
             where 
             c.Active == true 
             &&
             c.LastReviewedOn <= DateTime.Now.AddDays(-365) 
             &&
             ( // Owned by user
                // !ownerUserId.HasValue || 
                // c.OwnerUserId.Value == ownerUserId.Value
                ownerUserId.HasValue ? c.OwnerUserId.Value == ownerUserId.Value : true
             )
             &&
             ( // Owned by group
                // ownerGroupIds.Count == 0 ||
                // ownerGroupIds.Contains( c.OwnerGroupId.Value )
                ownerGroupIds.Count != 0 ? ownerGroupIds.Contains( c.OwnerGroupId.Value ) : true
             )
             select c ).Count();
}