I couldn’t figure out a way to perform the equivalent of WHERE Column1 IN ('A', 'B', 'C') in LINQ, where (‘A’, ‘B’, ‘C’) would represent an IEnumerable<T> where T is the type of Column1. So I thought it was an excellent time to write an extension method that would generate a dynamic expression tree that would add AND (Column1 == "A" OR Column1 = "B" OR ...) to the LINQ query. So I wrote the following code:

public static IQueryable<TSource> WhereIn<TSource, TKey> (
this IQueryable<TSource> source1,
Expression<Func<TSource, TKey>> keySelector,
IEnumerable<TKey> source2) {
if (null == source1)
throw new ArgumentNullException ("source1");
if (null == keySelector)
throw new ArgumentNullException ("keySelector");
if (null == source2)
throw new ArgumentNullException ("source2");
Expression where = null;
foreach (TKey value in source2) {
Expression equal = Expression.Equal (
Expression.Constant (value, typeof (TKey))
if (null == where)
where = equal;
where = Expression.OrElse (where, equal);
return source1.Where<TSource> (
Expression.Lambda<Func<TSource, bool>> (
where, keySelector.Parameters));

An example of the usage:

var q = (from u in db.Users
where u.LastLogin > new DateTime (2007, 5, 1)
orderby u.LastLogin descending
select new { u.FirstName, u.LastName, u.UserName, u.LastLogin }
).WhereIn (u => u.UserName, new string[] { "A", "B", "C" });

A day later, I found the right way that will actually generate “WHERE X IN (…)” in LINQ to SQL thanks to Mark Blomsma.

3 comments on “LINQ – WHERE X IN (…)

  1. Mark Blomsma’s solution has some limitations. If the size of the collection is too big, an exception occurs in RPC. I would segment the collection in 2000 items chunks if the size exceeds that limit.

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