LINQ Where in collection clause

Question

I've been looking on google but not finding anything that does the trick for me.

as you know SQL has a "where x in (1,2,3)" clause which allows you to check against multiple values. I'm using linq but I can't seem to find a piece of syntax that does the same as the above statement.

I have a collection of category id's (List) against which I would like to check

I found something that uses the .contains method but it doesn't even build.

Answer 1

You have to use the Contains method on your id list:

var query = from t in db.Table
            where idList.Contains(t.Id)
            select t;

Answer 2

The syntax is below:

IEnumerable<int> categoryIds = yourListOfIds;

var categories = _dataContext.Categories.Where(c => categoryIds.Contains(c.CategoryId));

The key thing to note is that you do the contains on your list of ids - not on the object you would apply the in to if you were writing sql.

Answer 3

Here's an article illustrating the approach. You should indeed use the Contains method over your collection which will be translated into IN clause.

Answer 4

Here is my realization of WhereIn() Method, to filter IQueryable collection by a set of selected entities:

 public static IQueryable<T> WhereIn<T,TProp>(this IQueryable<T> source, Expression<Func<T,TProp>> memberExpr, IEnumerable<TProp> values) where T : class
    {
        Expression predicate = null;
        ParameterExpression param = Expression.Parameter(typeof(T), "t");

        bool IsFirst = true;

        MemberExpression me = (MemberExpression) memberExpr.Body;
        foreach (TProp val in values)
        {
            ConstantExpression ce = Expression.Constant(val);


            Expression comparison = Expression.Equal(me, ce);

            if (IsFirst)
            {
                predicate = comparison;
                IsFirst = false;
            }
            else
            {
                predicate = Expression.Or(predicate, comparison);
            }
        }

        return predicate != null
            ? source.Where(Expression.Lambda<Func<T, bool>>(predicate, param)).AsQueryable<T>()
            : source;
    }

And calling of this method looks like:

IQueryable<Product> q = context.Products.ToList();

var SelectedProducts = new List<Product>
{
  new Product{Id=23},
  new Product{Id=56}
};
...
// Collecting set of product id's    
var selectedProductsIds = SelectedProducts.Select(p => p.Id).ToList();

// Filtering products
q = q.WhereIn(c => c.Product.Id, selectedProductsIds);