如何使用反射来调节多个属性以在linq的语句中检查平等，具体取决于传递的类别?[英] How use Reflection to condition multiple properties to check for equality in a LINQ .Where statement, depending on what class is passed?

问题描述

I'm trying to generalize a duplicate checker function, which depending on which type of object, checks the properties said class has (provided in a configuration) are equal to those in another list.

I have decided to create a Dictionary, which will accept a type string for the key (Book, Author, Shop, etc.) and an array of properties that need to be equal.

Example of Dictionary enties:

"Book", ["Title", "CoverImage", "NumberOfPages"] 
"Author", ["Name", "Address", "SomethingElse"]

Then, I pass an object to the function and use Reflection to get the name of the type...

obj.GetType().Name;

... which I then use to fetch the right KVP from the Dictionary, meaning that if I pass a Book object, I get "Book". We then use that to get the configuration via ...

configDictionary["obj.GetType().Name"]

... which gives us the array of strings that are the properties that we need to check equality on.

I've gotten to the part where I need something along the lines of

list.Where(x => --> for each of the strings in the array - x.GetType.GetProperty(string) && --> same for next string && same for next string

... and then I need to top it off with an...

x.Id != obj.Id

To make sure we check for duplicates based on our logic (different id's and matches on all properties but has different Id's thus - a duplicate).

The end query should look like

Books:

someList.Where(x => 
x.Title == obj.Title 
&& x.CoverImage == obj.CoverImage 
&& x.NumberOfPages == obj.NumberOfPages 
&& x.Id != obj.Id)
.FirstOrDefault();

Authors:

someList.Where(x => x.Name == obj.Name 
&& x.Address == obj.Address 
&& x.SomethingElse == obj.SomethingElse 
&& x.Id != obj.Id)FirstOrDefault();

推荐答案

Try to avoid reflection because it can slow down your application. As an alternative you can create a dictionary and put all comparators into it:

var configDictionary = new Dictionary<string, List<Func<object, object, bool>>>
{
    {
        "Book",
        new List<Func<object, object, bool>>
        {
            (b1, b2) => ((Book)b1).Title == ((Book)b2).Title,
            (b1, b2) => ((Book)b1).CoverImage == ((Book)b2).CoverImage,
            (b1, b2) => ((Book)b1).NumberOfPages == ((Book)b2).NumberOfPages,
            (b1, b2) => ((Book)b1).Id != ((Book)b2).Id,
        }
    },
    // same for Authors
};

Now you can use it in Where method:

var typeName = obj.GetType().Name; // here we using Reflection but once per collection, not per each item
var first = someList.Where(x => configDictionary[typeName].All(f => f(x, obj))).FirstOrDefault();

Also, because FirstOrDefault also has overload that accept predicate last line can be rewritten to:

var first = someList.FirstOrDefault(x => configDictionary[typeName].All(f => f(x, obj)));

其他推荐答案

A better solution will be creating custom attribute which will tag property. Then in class override default method Equals which will get all properties with this attribute and return equality.