为什么我不能在C＃中创建一个匿名类型的列表<T>？

Question

I am new to anonymous types in c#, and I want to create a list of anonymous types that contain 3 variables: string str , int num , DataTime time .

however, when I try to use the code from this question`s answers: A generic list of anonymous class it won't work for me.

I used a simple Console application to do it and I think I get the error because I don't have System.Core because someone in the comment of the question above said that:

(You also need a reference to System.Core of course.)

and I don't know what is System.Core and if I have it so it might be the problem

I do use Systme.Linq .

here is the code:

var list = new[] { str, num, time }.ToList();
list.add("hi", 5, DateTime.Now);

Console.WriteLine(list[0].num);

I also face issues when I try to specify the type of the variables for example string str .

Answer 1

You are missing some syntax. Anonymous types must be declared with new{...} . The property names must be declared when they cannot be inferred by a variable name. (You also have a typo in Add ; it should be uppercase).

The below works :

var str = "string";
var num = 5;
var time = DateTime.UtcNow;
// notice double "new" 
// property names inferred to match variable names
var list = new[] { new { str, num, time } }.ToList(); 

// "new" again. Must specify property names since they cannot be inferred
list.Add(new { str = "hi", num = 5, time = DateTime.Now });

Console.WriteLine(list[0].num);

With that said, this is pretty clunky. I'd suggest writing a class with the properties you want, or using ValueTuple .

This works and is clearer/cleaner:

var list = new List<(string str, int num, DateTime time)>();

// ValueTuple are declared in parens, method calls require parens as well
// so we end up with two sets of parens, both required 
list.Add((str, num, time));
list.Add(("hi", 5, DateTime.Now));

Console.WriteLine(list[0].num);

Another reason to prefer your own class or ValueTuple is that you cannot declare a method as accepting an anonymous type. In otherwords, something like this is not valid:

public void DoSomethingWithAnonTypeList(List<???> theList ) { ... } 

There is nothing* I can put to replace the ??? as anonymous types are both internal and have "unspeakable" names. You wouldn't be able to pass your list around and do something meaningful with it. So what's the point?

Conversely, I can declare a method as accepting a list of ValueTuple s:

public void DoSomethingWithTupleList(List<(string, int, DateTime)> theList) { 
     Console.WriteLine(theList[0].Item1);
} 

or using named tuples:

public void DoSomethingWithTupleList(List<(string str, int num, DateTime time)> theList) { 
     Console.WriteLine(theList[0].time);
} 

* You can technically pass your list of anonymous types to a generic method. However you won't be able to access the individual properties. The best you'd be able to do is access the list's Count or iterate over the list/enumerable and perhaps print the default ToString which doesn't really get you much either. There's not a generic constraint to help here. The third statement in this method will generate a compiler error :

public void DoSomethingGenerically<T>(List<T> theList) {

      Console.WriteLine(theList.Count); // valid
      Console.WriteLine(theList[0]); // valid, prints default ToString

      Console.WriteLine(theList[0].num); // invalid! What's the point?

}

var list = new[] { new { str = "hi", num = 5, time = DateTime.Now } }.ToList();
// valid due to type inference, but see comments above
DoSomethingGenerically(list); 

Do note that you'll have the same issue with ValueTuple , I'm just clarifying my "do nothing" statement.