Question
I have the following Entity
public class Person
{
public int PersonId { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
}
and have a list List<Person> badGuys
what I want to do is select from all persons except those in badGuys List
My Code
context.Persons
.where(p => !badGuys.Contain(p))
.ToList()
but I get an error
Only primitive types or enumeration types are supported in this context.
How to fix this?
Accepted Answer
You could make an array containing the ids of the bad guys and filter out those ids (which are of a primitive type, so it should work):
var badGuyIds = badGuys.Select(x => x.PersonId).ToArray();
context.Persons
.Where(p => !badGuyIds.Contain(p.PersonId))
.ToList();
Popular Answer
You can implement your own method to make necessary expression tree, like here:
public static IQueryable<TEntity> WhereNotIn<TEntity, TValue>(
this IQueryable<TEntity> queryable,
Expression<Func<TEntity, TValue>> valueSelector,
IEnumerable<TValue> values)
where TEntity : class
{
if (queryable == null)
throw new ArgumentNullException("queryable");
if (valueSelector == null)
throw new ArgumentNullException("valueSelector");
if (values == null)
throw new ArgumentNullException("values");
if (!values.Any())
return queryable.Where(e => true);
var parameterExpression = valueSelector.Parameters.Single();
var equals = from value in values
select Expression.NotEqual(valueSelector.Body, Expression.Constant(value, typeof (TValue)));
var body = equals.Aggregate(Expression.And);
return queryable.Where(Expression.Lambda<Func<TEntity, bool>>(body, parameterExpression));
}
}
And now you can call this extension method
var badGuys = new int[] { 100, 200, 300 };
context.Persons.WhereNotIn(p => p.PersionId, badGuys);
This method makes the same thing as:
context.Persons.Where(p => p.PersonId != badGuys[0]
&& p.PersonId != badGuys[1]
. . .
&& p.PersonId != badGuys[N]);
for each element of badGuys enumerable object.
An additional advantage of the method is its universality, cause you can call it for any property of any entity, f.e. context.Persons.WhereNotIn(p => p.LastName, new[] { "Smith", "Brown", "Jones" }).
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