Expression<Func<T, bool>>与Func<T, bool>的区别

https://www.cnblogs.com/wow-xc/articles/4952233.html

Expression<Func<TObject, bool>>与Func<TObject, bool>的区别

Func<TObject, bool>是委托(delegate)

Expression<Func<TObject, bool>>是表达式

Expression编译后就会变成delegate，才能运行。比如

Expression<Func<int, bool>> ex = x=>x < 100;

Func<int, bool> func = ex.Compile(); 

然后你就可以调用func：

func(5) //-返回 true

func(200) //- 返回 false

而表达式是不能直接调用的。

//错误的代码
Func<QuestionFeed, bool> predicate = null;

if (type == 1)
{
    predicate = f => f.FeedID == id && f.IsActive == true;
}
else
{
    predicate = f => f.FeedID == id;
}
//_questionFeedRepository.Entities的类型为IQueryable<QuestionFeed>
_questionFeedRepository.Entities.Where(predicate);

上面代码逻辑是根据条件动态生成LINQ查询条件，将Func类型的变量作为参数传给Where方法。

实际上Where要求的参数类型是：Expression<Func<TSource, bool>>。

解决方法：

不要用Func<TSource, bool>，用Expression<Func<TSource, bool>>。

//正确的代码
Expression<Func<QuestionFeed, bool>> predicate=null;
if (type == 1)
{
    predicate = f => f.FeedID == id && f.IsActive == true;
}
else
{
    predicate = f => f.FeedID == id;
}
_questionFeedRepository.Entities.Where(predicate);