c# - 动态创建表达式调用方法 EntityFunctions.DiffDays

Question

我正在尝试动态创建以下 Where 子句表达式:

context.Cars.
Where(c => EntityFunctions.DiffDays(c.Created, c.Created) == null).
ToList()

这是我用来创建表达式的代码:

var parameter = Expression.Parameter(typeof(Car), "c");
var property = Expression.Property(parameter, "Created");
var function = Expression.Call(typeof(EntityFunctions), "DiffDays", 
    null, property, property);
var comparison = Expression.Equal(function, Expression.Constant(null));
var result = Expression.Lamda<Func<Car, bool>>(comparison, parameter);

结果显示(似乎缺少“EntityFunctions.DiffDays”):

{c => (DiffDays(c.Created, c.Created) == null)}

当我尝试执行时:

context.Cars.Where(result.Compile()).ToList()

我收到错误信息:

this function can only be invoked from linq to entities

你知道我错过了什么吗？是不是因为它只显示“DateDiff”而不是“EntityFunctions.DateDiff”

谢谢。亚当

Answer 1

删除最后一行的“Compile()”调用，如下所示:

context.Cars.Where(result).ToList();

并将编译推迟到 LinqToEntities。