从多个参数构建IQueryable查询(EF4)

Question

我正在尝试从List<Expression<Func<T, bool>>>形式的谓词列表中构建一个"Or“谓词

 public static IQueryable<T> Search<T>(this IQueryable<T> source, List<Expression<Func<T, bool>>> predicates = null)
            where T : EntityObject
        {
            if (predicates == null || predicates.Count == 0)
                return source;
            else if (predicates.Count == 1)
                return source.Where(predicates[0]);
            else
            {
                var row = Expression.Parameter(typeof(T), "row");
                var compoundExpression = predicates[0];

                for (int i = 1; i < predicates.Count; i++)
                {
                    compoundExpression = compoundExpression.Or(predicates[i]);
                }
                return source.Where(compoundExpression);
            }
        }

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

但是这会返回我源代码中的每一行吗？

为了进行测试，我正在寻找c=>c.FullName.Contains("Smith") or c=>c.FullName.Contains("Jones")

我尝试修改为使用PredicateBuilder，但它仍然返回源代码中的每一行。

public static IQueryable<T> Search<T>(this IQueryable<T> source, List<Expression<Func<T, bool>>> predicates = null)
            where T : EntityObject
        {
            if (predicates == null || predicates.Count == 0)
                return source;
            else if (predicates.Count == 1)
                return source.Where(predicates[0]);
            else
            {
                var pb = PredicateBuilder.False<T>();
                for (int i = 0; i < predicates.Count; i++)
                {
                    pb = pb.Or(predicates[i]);
                }
                return source.AsExpandable().Where(pb);
            }
        }

非常感谢您的帮助！

最终结果将是既允许AND也允许OR

例如c=>c.FullName.Contains("Dav")和c=>c.CustomerType == 'Staff‘

Answer 1

有时周末会对代码做一些奇怪的事情！

不知道我做了什么不同，但这是有效的：

public static IQueryable<T> Search<T>(this IQueryable<T> source, List<Expression<Func<T, bool>>> predicates = null)
            where T : EntityObject
        {
            if (predicates == null || predicates.Count == 0)
                return source;
            else if (predicates.Count == 1)
                return source.Where(predicates[0]);
            else
            {
                var query = PredicateBuilder.False<T>();
                foreach (var predicate in predicates)
                {
                    query = query.Or(predicate);
                }

                return source.AsExpandable().Where(query);
            }
        }

PredicateBuilder看起来是一小段很棒的代码。

Answer 2

试一试

public static IQueryable<T> Search<T>(this IQueryable<T> source, IEnumerable<Expression<Func<T, bool>>> predicates = null)
        where T : EntityObject
    {
        if (predicates == null || !predicates.Any())
            return source;
        else
        {
             ParameterExpression p = Expression.Parameter(typeof(T), "p");
             Expression<Func<T,Bool>> predicate = 
                        Expression.Lambda<Func<T,Bool>(
                                   predicates.Select(l => ReParameteriser(l.Body, l.Paramaters[0], p)
                                             .Aggregate((b1,b2) => Expression.Or(b1,b2)),
                                   new ParamaterExpression[]{p});
            return source.Where(predicate);
        }
   }

public class ReParameteriser : ExpressionVisitor
{
    ParameterExpression originalParameter; 
    ParameterExpression newParameter;

    private ReParameteriser(){}
    protected ReParameteriser (ParameterExpression originalParameter, ParameterExpression newParameter) 
    {
         this.originalParameter = originalParameter;
         this.new = newParameter;
    }

    public static Expression ReParameterise(Expression expression, ParameterExpression originalParameter, ParameterExpression newParameter)
    {
        return new ReParameteriser(original,newParameter).Visit(expression);
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        if (node == originalParameter)
            return newParameter;
        else
            return node;
    }
}

注意: ExpressionVisitor类是.Net4类，因此如果您想要针对较早的环境，则需要编写您自己的环境。这方面的代码只需谷歌搜索一下，但常用的资源是Matt Warren的博客http://blogs.msdn.com/b/mattwar/archive/2008/11/18/linq-links.aspx