如何将表达式转换为 lambda 表达式？

Question

I am trying to convert an Expression of type Expression<Func<Entity, bool>> to a Func<Entity, bool> .

The background here is that I am trying to mock a repository so that it will return one of a collection of mock entities for a given key value. (I could hard code the input values to the mocked method but this seems like the wrong approach for a large number of items).

So I am trying to mock the First method on my repository like this:

var collection = new List<Entity> 
{ 
    mockedEntity1, 
    mockedEntity2, 
    mockedEntity3, 
    ... 
};

mockRepository
    .Setup(rep => rep.First(It.IsAny<Expression<Func<Entity, bool>>>()))
    .Returns<Expression<Func<Entity, bool>>>(e => collection.First(e));

This doesn't work because collection.First takes a Func rather than an Expression>. So I have got to the point where I need to convert the Expression to the Func that it contains.

Perhaps there a simpler or better to do this?

Answer 1

You need to call Compile on the expression.

Answer 2

It already is a lambda expression. But to get a delegate from the lambda, call .Compile() .

In the general sense - to make a lambda from an Expression you would use Expression.Lambda, indicating the desired type and including the parameter (declaration) instances (from Expression.Parameter). However, this is not required here.