Question

我知道如何构建表达式树来查询实体框架，例如。

        string propertyName = "Name";
        string keyword = "Methanol";

        ParameterExpression parameter = Expression.Parameter(typeof(Chemical), "x");

        MemberExpression me = Expression.Property(parameter, propertyName);
        ConstantExpression constant = Expression.Constant(keyword, typeof(string));
        BinaryExpression body = Expression.Equal(me, constant);
        var isEqualExpressionTree = Expression.Lambda<Func<Chemical, bool>>(body, new[] { parameter });
        Expression<Func<Chemical, bool>> funcExpression = (Expression<Func<Chemical, bool>>)isEqualExpressionTree;

        using (var mLEntities = new myLab02Entities1())
        {
            var cl = mLEntities.Chemicals.AsQueryable().Where(funcExpression).ToList();       
            return cl;

        }

“名称”是SQL数据库中的一个字段，应检查“甲醇”。内置的Lambda是： {x =＆gt; （x.Name ==“甲醇”）}。现在我想测试数据库的DateTime字段，但只测试Date-section。所以我的Lambda将是

{x =＆gt; DbFunctions.TruncaateTime（x.EntryDate）== DbFunction.TruncateTime（testDate）}。这是按原样工作的，但是如何将其转换为表达式？谢谢，哈基

Answer 1

 string propertyName = "EntryDate";
 DateTime testDate = DateTime.Now;            

 ParameterExpression parameter = Expression.Parameter(typeof(Chemical), "x");
 MemberExpression me = Expression.Property(parameter, propertyName);
 var ce = Expression.Convert(me, typeof(DateTime?));
 MethodCallExpression mc = Expression.Call(null, typeof(DbFunctions).GetMethod("TruncateTime", new Type[] { typeof(DateTime?) }), ce);
 ConstantExpression constant = Expression.Constant(testDate, typeof(DateTime?));
 BinaryExpression body = Expression.Equal(mc, constant);
 var isEqualExpressionTree = Expression.Lambda<Func<Chemical, bool>>(body, new[] { parameter });
 Expression<Func<Chemical, bool>> funcExpression = (Expression<Func<Chemical, bool>>)isEqualExpressionTree;

如何在表达式中使用DateTime for LINQ to Entities？

1 个答案: