C#, Linq2Sql: Is it possible to concatenate two queryables into one?

Answer 1

You have two options here - Queryable.Union, or expression combination. I'd generally favor the latter, via OrElse - which (with LINQ-to-SQL at least) you can do with 2 expressions (see below) - but in either case it should get composed:

    using(var ctx = new DataClasses1DataContext())
    {
        ctx.Log = Console.Out;
        Expression<Func<Customer, bool>> lhs =
            x => x.Country == "UK";
        Expression<Func<Customer, bool>> rhs =
            x => x.ContactName.StartsWith("A");

        var arr1 = ctx.Customers.Where(
            lhs.OrElse(rhs)).ToArray();

        var arr2 = ctx.Customers.Where(lhs)
            .Union(ctx.Customers.Where(rhs)).ToArray();
    }

Both arr1 and arr2 each only perform 1 database hit (although the TSQL is different; the first has an OR in the WHERE clause; the second has two separate queries with UNION).

Here's the extension method I used:

static Expression<Func<T, bool>> OrElse<T>(
    this Expression<Func<T, bool>> lhs,
    Expression<Func<T, bool>> rhs)
{
    var row = Expression.Parameter(typeof(T), "row");
    var body = Expression.OrElse(
        Expression.Invoke(lhs, row),
        Expression.Invoke(rhs, row));
    return Expression.Lambda<Func<T, bool>>(body, row);
}