T，T，BOOL>从Func键&LT获取; + T实例表达式来; Func键< T，BOOL>> [英] Getting from Func<T, T, bool> + T instance to Expression<Func<T, bool>>

问题描述

如果我有一个 Func键< T，T，BOOL> ，告诉我如何在两个TS比较如

If I have a Func<T, T, bool> that tells me how to compare two Ts as in

var comparer = (a, b) => 
    a.IdPart1 == b.IdPart1 && a.IdPart2 == b.IdPart2;

和我想用这个和T的具体实例来创建表达式来; Func键< T，BOOL>> 用作谓语为凡（），如

And I want to use this and a concrete instance of T to create a Expression<Func<T, bool>> to be used as a predicate for Where() such as

T instance = GetSomeT();
TRepository.GetAll().Where(x => 
    x.IdPart1 == instance.IdPart1 && x.IdPart2 == instance.IdPart2);

但当然，动态的，这样我可以代替写

but of course dynamically so that I could instead write

var predicate = something depending on comparer and instance;
TRepository.GetAll().Where(predicate);

这可能吗？

Is it possible?

更酷是，如果我可以动态创建谓词只是在T与KeyAttribute和和之间的所有属性之间的平等，这样我就不需要比较器，只需T.

Even cooler would be if I could dynamically create the predicate just from T with equals between all properties with KeyAttribute and and in between, so that I wouldn't need the comparer, just T.

时的是的可能吗？ ：）

推荐答案

假设你可以得到比较器为表达式来; Func键< T，T，BOOL>> ，那么你可以使用的 LINQKit 来做到这一点。

Assuming you can get comparer as Expression<Func<T, T, bool>>, then you can use LINQKit to do this.

是这样的：

Expression<Func<Foo, Foo, bool>> comparer =
    (a, b) => a.IdPart1 == b.IdPart1 && a.IdPart2 == b.IdPart2;

Foo instance = …;

Expression<Func<Foo, bool>> predicate = x => comparer.Invoke(x, instance);
var expandedPredicate = predicate.Expand();

var result = FooRepository.GetAll().Where(expandedPredicate);

的

展开（）部分是很重要的，因为这是LINQKit的神奇的发生。稍微不同的语法做同样的事情，就是用 AsExpandable（）而不是：

The Expand() part is important, because that's where LINQKit's "magic" happens. A slightly different syntax to do the same thing is to use AsExpandable() instead:

var result = FooRepository.GetAll().AsExpandable().Where(predicate);

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