Dynamic Linq Expression for IEnumerable<int>.contains(MemberExpression)

Question

I want to create an expression using the Linq Expression tree to simulate this:

List<int> ids = new List<int>();

// Fill ids with data

db.Where(a => ids.Contains(a.Id));

This is where I have gotten, but I am still missing something:

MemberExpression me = Expression.Property(pe, typeof(T).GetProperty(property));

Expression callContains = Expression.Call(typeof(System.Linq.Enumerable), "Contains", new Type[] { me.Type }, me);

How can I do what I want to do properly?

Answer 1

Because Contains is an extension method, you'll also have to provide the ids collection as a parameter, in this case, as a ConstantExpression.

Your implementation might be slightly different, but it would look a bit like this:

public static IQueryable<T> DynamicContains<T, TProperty>(
    this IQueryable<T> query, 
    string property, 
    IEnumerable<TProperty> items)
{
    var pe = Expression.Parameter(typeof(T));
    var me = Expression.Property(pe, property);
    var ce = Expression.Constant(items); 
    var call = Expression.Call(typeof(Enumerable), "Contains", new[] { me.Type }, ce, me);
    var lambda = Expression.Lambda<Func<T, bool>>(call, pe);
    return query.Where(lambda);
}

db.DynamicContains("Id", ids);