Use dictionary inside linq query

Question

I want to use some dictionary inside my linq query, however since LINQ to entities can't translate the use of the dictionary it throws an exception. Actually the same problem was described in the following question: linq to entity framework: use dictionary in query

I wasn't satisfied of the solution which was described there. I'm sure there is some other solution for this problem. I don't want to use the ToList/ToArray method - which will bring all data to memory.

What is the best way to solve this issue, without pulling the db data to memory?

Answer 1

Your example looks like you actually don't need the functionality of a dictionary, you simply want to have WHERE IN functionality.
You could use the following to achieve this:

var countries = Countries.WhereIn(x => x.CountryId, dict.Keys);

WhereIn is not a built in query operator, you need to write it yourself - or copy:

/// <summary>
/// Holds extension methods that simplify querying.
/// </summary>
public static class QueryExtensions
{
    /// <summary>
    ///   Return the element that the specified property's value is contained in the specified values.
    /// </summary>
    /// <typeparam name="TElement"> The type of the element. </typeparam>
    /// <typeparam name="TValue"> The type of the values. </typeparam>
    /// <param name="source"> The source. </param>
    /// <param name="propertySelector"> The property to be tested. </param>
    /// <param name="values"> The accepted values of the property. </param>
    /// <returns> The accepted elements. </returns>
    public static IQueryable<TElement> WhereIn<TElement, TValue>(
        this IQueryable<TElement> source, 
        Expression<Func<TElement, TValue>> propertySelector, 
        params TValue[] values)
    {
        return source.Where(GetWhereInExpression(propertySelector, values));
    }

    /// <summary>
    ///   Return the element that the specified property's value is contained in the specified values.
    /// </summary>
    /// <typeparam name="TElement"> The type of the element. </typeparam>
    /// <typeparam name="TValue"> The type of the values. </typeparam>
    /// <param name="source"> The source. </param>
    /// <param name="propertySelector"> The property to be tested. </param>
    /// <param name="values"> The accepted values of the property. </param>
    /// <returns> The accepted elements. </returns>
    public static IQueryable<TElement> WhereIn<TElement, TValue>(
        this IQueryable<TElement> source, 
        Expression<Func<TElement, TValue>> propertySelector, 
        IEnumerable<TValue> values)
    {
        return source.Where(GetWhereInExpression(propertySelector, values.ToList()));
    }

    /// <summary>
    ///   Gets the expression for a "where in" condition.
    /// </summary>
    /// <typeparam name="TElement"> The type of the element. </typeparam>
    /// <typeparam name="TValue"> The type of the value. </typeparam>
    /// <param name="propertySelector"> The property selector. </param>
    /// <param name="values"> The values. </param>
    /// <returns> The expression. </returns>
    private static Expression<Func<TElement, bool>> GetWhereInExpression<TElement, TValue>(
        Expression<Func<TElement, TValue>> propertySelector, ICollection<TValue> values)
    {
        var p = propertySelector.Parameters.Single();
        if (!values.Any())
            return e => false;

        var equals =
            values.Select(
                value =>
                (Expression)Expression.Equal(propertySelector.Body, Expression.Constant(value, typeof(TValue))));
        var body = equals.Aggregate(Expression.OrElse);

        return Expression.Lambda<Func<TElement, bool>>(body, p);
    }
}