尝试将IList转换为List以进行AddRange

Question

I have a IList . I try to call ToList and then AddRange .

However, the ToList() overwrite all the results. How come?

private void AddAliasesThatContainsCtid(string ctid, IList<MamConfiguration_V1> results)
{

...
    foreach (var alias in aliases)
    {
        var aliasId = "@" + alias;

    results.ToList().AddRange(mMaMDBEntities.MamConfigurationToCTIDs_V1.Where(item => item.CTID == aliasId)
                             .Select(item => item.MamConfiguration_V1)
                             .ToList());
    }

}

Answer 1

.ToList() does not convert an IEnumerable<T> to a List<T> , it creates and returns a new list filled with the values of the enumerable.

So your result.ToList() will create a new list and fill it with some data. But it will not change the contents of the object referenced by the result parameter.

In order to actually change the contents of the result parameter you have to use the .Add method of it or if your design allows it change the type of result to List<..> .

Answer 2

Your code is equivalent:

// Create new List by calling ToList()
var anotherList = results.ToList();
anotherList.AddRange(...);

So, you actually add items into anotherList , not result list at all.

To get the correct result, two approaches:

1:

Declare results as out and assign back:

results = anotherList;

Or:

results = results.ToList().AddRange(...)

2:

Use Add method supported by IList instead of AddRange

Answer 3

It's simple:

public static class ListExtensions
{
    public static IList<T> AddRange<T>(this IList<T> list, IEnumerable<T> range)
    {
        foreach (var r in range)
        {
            list.Add(r);
        }
        return list;
    }
}

Answer 4

Although IList<T> doesn't have an AddRange() , it does have Add() , so you could write an extension method for IList<T> that would let you add a range to it.

If you did, your code would become:

private void AddAliasesThatContainsCtid(string ctid, IList<MamConfiguration_V1> results)
{
...
    results.AddRange(mMaMDBEntities.MamConfigurationToCTIDs_V1
        .Where(item => item.CTID == aliasId)
        Select(item => item.MamConfiguration_V1));
   }
}

Compilable sample implementation:

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

namespace Demo
{
    internal class Program
    {
        static void Main()
        {
            IList<string> list = new List<string>{"One", "Two", "Three"};
            Print(list);
            Console.WriteLine("---------");

            var someMultiplesOfThree = Enumerable.Range(0, 10).Where(n => (n%3 == 0)).Select(n => n.ToString());
            list.AddRange(someMultiplesOfThree); // Using the extension method.

            // Now list has had some items added to it.
            Print(list);
        }

        static void Print<T>(IEnumerable<T> seq)
        {
            foreach (var item in seq)
                Console.WriteLine(item);
        }
    }

    // You need a static class to hold the extension method(s):

    public static class IListExt
    {
        // This is your extension method:

        public static IList<T> AddRange<T>(this IList<T> @this, IEnumerable<T> range)
        {
            foreach (var item in range)
                @this.Add(item);

            return @this;
        }
    }
}