在匿名方法、lambda 表达式、查询表达式或本地 function 中使用的通用参数类型

Question

I am new to C# and I am trying to create a method that accept generic parameter type like in Javascript.

Let says I have a List of User objects with name and id properties.

In Javascript, to search through the list using property value will be as simple as:

 class User{ constructor(name, id){ this.name = name; this.id = id; } } let users = [ new User("John",1), new User("Deer", 2), new User("Bruce", 3), ] function findArr(arr, propertyName, propertyValue){ for(let a of arr) if(a[propertyName] === propertyValue) return a; return null; } console.log(findArr(users, "name", "Deer")) console.log(findArr(users, "id", 3))

Then, I try to recreate this in C#:

using System;
using System.Collections.Generic;
using System.Linq;

public class User{
    public string name {get; set;}
    public int id {get; set;}
}

public class HelloWorld
{
    public static void Main(string[] args)
    {
        List<User> Users = new List<User>{
            new User{name="John", id=1},
            new User{name="Deer", id=2},
            new User{name="Bruce", id=3}
        };
        
        int propertyValue = 2;
        var u = FindList<User, int>(Users, "id", ref propertyValue);
        Console.WriteLine(u.name + " " + u.id);
    }
    
    public static T FindList<T, K>(List<T> obj, string propertyName, ref K propertyValue){
        return (T)obj.FirstOrDefault(o=>
                (K)o.GetType().GetProperty(propertyName).GetValue(o, null) 
                == 
                propertyValue
            );
    }
}

But this will throw exception: Cannot use ref, out, or in parameter 'propertyValue' inside an anonymous method, lambda expression, query expression, or local function .

Why is this exception thrown?

How do I make this work?

Answer 1

As the error message states, you cannot use the ref or out keywords in an anonymous function. This is by design, as detailed in this answer .

The good news is, you do not need to use ref at all. You can achieve what you want simply by passing the property value as a normal parameter, like so:

public static T FindList<T, K>(List<T> obj, string propertyName, K propertyValue){
    return (T)obj.FirstOrDefault(o=>
EqualityComparer<K>.Default.Equals((K)o.GetType().GetProperty(propertyName).GetValue(o, null) , propertyValue));
}

Note that the equality check with == operator won't work for unconstrained generic type in general, so we use the EqualityComparer method instead.

Answer 2

Javascript is a weakly typed language, every object is essentially just a dictionary.

If you ever find yourself accessing properties by name in C#, you're probably doing something "wrong".

Rather than using reflection, you could pass in a function to access the property;

    public static T FindList<T, V>(List<T> obj, Func<T,V> propertyAccess, V propertyValue){
        return obj.FirstOrDefault(o => propertyAccess(o) == propertyValue);
    }

var u = FindList<User, int>(Users, u => u.id, propertyValue);

However, I don't think this method really adds anything to your C# code. You'd be better off just writing the straight forward solution, without complicating anything with your own "helper" method;

var u = Users.FirstOrDefault(u => u.id == propertyValue);

It's not the same as the javascript code you are familiar with. But you're writing in C# now. If you want to work with other C# programmers, you will need to change the way you think about programming problems.