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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?

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1 に答える 1

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あなたの例は、実際には辞書の機能を必要としないように見えます。単に機能が必要なだけですWHERE IN。
これを実現するには、次を使用できます。

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

WhereIn組み込みのクエリ演算子ではありません。自分で記述するか、コピーする必要があります。

/// <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);
    }
}
于 2012-11-22T14:39:58.787 に答える