我一直在使用Expression树在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，我可以通过过滤器集合并起作用，但是当我看一下SQL语句时，它是查询整个表并将筛选器应用于结果集合，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
}
}
要调用此类，我要做这样的事情：
var whereDeleg = QueryExpressionBuilder.GetExpression<Tax>(filters).Compile();
var myList = _dbContext.MyEntity.Where(whereDeleg).ToList();
我传递的滤镜参数是此类的集合：
public class Filter
{
public string PropertyName { get; set; }
public Op Operation { get; set; }
public object Value { get; set; }
}
我将不胜感激。
答案 0 :(得分：4)
主要问题不是类，而是使用方法：
var whereDeleg = QueryExpressionBuilder.GetExpression<Tax>(filters).Compile();
var myList = _dbContext.MyEntity.Where(whereDeleg).ToList();
您正在从方法中提取Expression<Func<T, bool>>，但是随后Complie()调用将其转换为Func<T, bool>。因此，尽管_dbContext.MyEntity是IQueryable<T>，但是没有IQueryable<T>扩展方法Where占用Func<T, bool>（它们全部占用Expression<Func<T, bool>>）。但是由于IQueryable<T>继承了IEnumerable<T>（因此是Where），因此编译器为IEnumerable<T>（在Enumerable类中定义）使用Where扩展方法。
这将使Where（以及以下所有方法，如果有的话）在IQueryable<T>之前（在您的情况下是整个表格）执行并具体化查询之后，才执行客户端。
Returning IEnumerable<T> vs. IQueryable<T>涵盖了IEnumerable<T>和IQueryable<T>之间的差异。您需要做的是确保始终使用IEnumerable<T>而不是使用相同名称和类似 looking 参数的Expression<Func<...>>扩展方法而不是Func<...>方法Compile。
话虽如此，您应该直接使用方法结果，而无需调用var predicate = QueryExpressionBuilder.GetExpression<Tax>(filters);
var myList = _dbContext.MyEntity.Where(predicate).ToList();
：
var myList = _dbContext.MyEntity.Where(QueryExpressionBuilder.GetExpression<Tax>(filters)).ToList();
或者只是
QueryExpressionBuilder甚至更好的是，将以下自定义扩展方法添加到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;
}
类中：
var myList = _dbContext.MyEntity.Where(filters).ToList();
能够简单地使用（并最大程度地减少犯错误的可能性）：
filters侧面说明：主表达式构建器方法的实现过于复杂，并且还会破坏传递的输入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;
}
列表。可以简化如下（没有上述缺陷）：
$datas = Schools::orderBy('id','desc')
->whereBetween('created_at', ['2018-06-27', '2018-06-28'])
->get()