EF Core 动态滤波器

Question

我一直在使用表达式树的 EF Core 查询的动态过滤器类中工作，一切看起来都很好，过滤器正在工作，我可以传递一个过滤器集合并且它可以工作，但是当我查看 SQL 语句时，它正在查询整个表并对结果集合应用过滤器，这是我的课程......

   public static class QueryExpressionBuilder
{
    private static readonly MethodInfo ContainsMethod = typeof(string).GetMethod("Contains", new[] { typeof(string) });
    private static readonly MethodInfo StartsWithMethod = typeof(string).GetMethod("StartsWith", new[] { typeof(string) });
    private static readonly MethodInfo EndsWithMethod = typeof(string).GetMethod("EndsWith", new[] { typeof(string) });

    #region DynamicWhere

    /// <summary>Where expression generator.</summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="filters">The filters.</param>
    /// <returns></returns>
    public static Expression<Func<T, bool>> GetExpression<T>(IList<Filter> filters)
    {
        if (filters.Count == 0)
            return null;

        ParameterExpression param = Expression.Parameter(typeof(T), "t");
        Expression exp = null;

        if (filters.Count == 1)
            exp = GetExpression(param, filters[0]);
        else if (filters.Count == 2)
            exp = GetExpression<T>(param, filters[0], filters[1]);
        else
        {
            while (filters.Count > 0)
            {
                var f1 = filters[0];
                var f2 = filters[1];

                if (exp == null)
                    exp = GetExpression<T>(param, filters[0], filters[1]);
                else
                    exp = Expression.AndAlso(exp, GetExpression<T>(param, filters[0], filters[1]));

                filters.Remove(f1);
                filters.Remove(f2);

                if (filters.Count == 1)
                {
                    exp = Expression.AndAlso(exp, GetExpression(param, filters[0]));
                    filters.RemoveAt(0);
                }
            }
        }

        return Expression.Lambda<Func<T, bool>>(exp, param);
    }

    /// <summary>Comparision operator expression generator.</summary>
    /// <param name="param">The parameter.</param>
    /// <param name="filter">The filter.</param>
    /// <returns></returns>
    private static Expression GetExpression(ParameterExpression param, Filter filter)
    {
        MemberExpression member = Expression.Property(param, filter.PropertyName);
        var type = member.Type;
        ConstantExpression constant;
        switch (type.Name)
        {
            case "Int32":
                constant = Expression.Constant(Convert.ToInt32(filter.Value));
                break;
            case "String":
            default:
                constant = Expression.Constant(filter.Value);
                break;
        }

        // ConstantExpression constant = Expression.Constant(filter.Value);

        switch (filter.Operation)
        {
            case Op.Equals:
                return Expression.Equal(member, constant);

            case Op.GreaterThan:
                return Expression.GreaterThan(member, constant);

            case Op.GreaterThanOrEqual:
                return Expression.GreaterThanOrEqual(member, constant);

            case Op.LessThan:
                return Expression.LessThan(member, constant);

            case Op.LessThanOrEqual:
                return Expression.LessThanOrEqual(member, constant);

            case Op.Contains:
                return Expression.Call(member, ContainsMethod, constant);

            case Op.StartsWith:
                return Expression.Call(member, StartsWithMethod, constant);

            case Op.EndsWith:
                return Expression.Call(member, EndsWithMethod, constant);
        }

        return null;
    }

    /// <summary>And logic connector expression generator.</summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="param">The parameter.</param>
    /// <param name="filter1">The filter1.</param>
    /// <param name="filter2">The filter2.</param>
    /// <returns></returns>
    private static BinaryExpression GetExpression<T>(ParameterExpression param, Filter filter1, Filter filter2)
    {
        var bin1 = GetExpression(param, filter1);
        var bin2 = GetExpression(param, filter2);

        return Expression.AndAlso(bin1, bin2);
    }

    #endregion

}

}

为了调用这个类，我做了这样的事情：

var whereDeleg = QueryExpressionBuilder.GetExpression<Tax>(filters).Compile();
var myList = _dbContext.MyEntity.Where(whereDeleg).ToList();

我传递的过滤器参数是此类的集合：

public class Filter
{
    public string PropertyName { get; set; }
    public Op Operation { get; set; }
    public object Value { get; set; }
}

我将不胜感激任何帮助。

Answer 1

主要问题不是类，而是你使用它的方式：

var whereDeleg = QueryExpressionBuilder.GetExpression<Tax>(filters).Compile();
var myList = _dbContext.MyEntity.Where(whereDeleg).ToList();

您正在从您的方法中获取Expression<Func<T, bool>>，但随后Complie()调用将其转换为Func<T, bool>. 因此，尽管_dbContext.MyEntity是IQueryable<T>，但没有IQueryable<T>扩展方法Where采用Func<T, bool>（它们采用的全部Expression<Func<T, bool>>）。但由于IQueryable<T>继承（因此是a）IEnumerable<T>，编译器找到并使用Where扩展方法for IEnumerable<T>（在类中定义Enumerable）。

这使得Where（以及所有以下方法，如果有的话）在执行和具体化之前的查询Where（在您的情况下 - 整个表）之后执行客户端。

Returning IEnumerable<T> vs. IQueryable<T>涵盖了IQueryable<T>和之间的区别。您所需要的只是确保始终通过使用而不是调用扩展方法而不是具有相同名称和类似外观参数的方法。IEnumerable<T>IQueryable<T>IEnumerable<T>Expression<Func<...>>Func<...>

话虽如此，您应该直接使用方法结果而不调用Compile：

var predicate = QueryExpressionBuilder.GetExpression<Tax>(filters);
var myList = _dbContext.MyEntity.Where(predicate).ToList();

要不就

var myList = _dbContext.MyEntity.Where(QueryExpressionBuilder.GetExpression<Tax>(filters)).ToList();

或者更好的是，将以下自定义扩展方法添加到类中QueryExpressionBuilder：

public static IQueryable<T> Where<T>(this IQueryable<T> source, IList<Filter> filters)
{
    var predicate = GetExpression<T>(filters);
    return predicate != null ? source.Where(predicate) : source;
}

能够简单地使用（并最大限度地减少犯错误的机会）：

var myList = _dbContext.MyEntity.Where(filters).ToList();

旁注：主要表达式生成器方法实现过于复杂，并且还会破坏传递的输入filters列表。可以简化如下（不存在上述缺陷）：

public static Expression<Func<T, bool>> GetExpression<T>(IEnumerable<Filter> filters)
{
    var param = Expression.Parameter(typeof(T), "t");
    var body = filters
        .Select(filter => GetExpression(param, filter))
        .DefaultIfEmpty()
        .Aggregate(Expression.AndAlso);
    return body != null ? Expression.Lambda<Func<T, bool>>(body, param) : null;
}