Covariance/Conversion issue with Generics

Question

Consider the code:

using System.Collections.Generic;
namespace TestingTypes
{
    class Program
    {
        static void Main(string[] args)
        {
            var strings = new List<string>();               
            INeedToPassThisMethodAListOfObjects(strings as List<object>);
        }

        static void INeedToPassThisMethodAListOfObjects(List<object> objects) { }
    }
}

1>------ Build started: Project: TestingTypes, Configuration: Debug Any CPU ------
1>c:\users\[censored]\TestingTypes\Program.cs(9,41,9,64): error CS0039: Cannot convert 
type 'System.Collections.Generic.List<string>' to 
'System.Collections.Generic.List<object>' via a reference conversion, boxing 
conversion, unboxing conversion, wrapping conversion, or null type conversion

You might say that a List<string> is a List<object>, since a string is an object and C# 4 is supposed to support covariance in generic types.

Why does the compiler say it can't convert the type?
How do I pass "strings" to the method?

Justin Niessner · Answer 1 · 2012-10-24T17:10:00.023

3

You're getting the error because a List<string> is not a List<object>.

You can call list.Add(new TextBox()) on List<object> but, obviously, the same call doesn't work on List<string>.

C# Generics only allow for Covariance if the generic type is immutable (which is my the cast from List to IEnumerable works).

If you need to pass the list to a method, you could try passing

listOfStrings.Cast<object>();

On the downside, if you use that solution, any modifications made to the list inside the method call will not be reflected in the original list (because the call to Cast creates a new list).

If you have control of the method INeedToPassThisMethodAListOfObjects and that method only needs to iterate over the collection rather than modify it, you can change the parameter type to be IEnumerable<object> in which case you'd simply be able to pass your List<string> without any issues.

edited Oct 24 '12 at 17:10

answered Oct 24 '12 at 17:04

Justin Niessner

242,243
40
408
536

Yeah, it kind of is: http://blogs.msdn.com/b/csharpfaq/archive/2010/02/16/covariance-and-contravariance-faq.aspx – James Orr Oct 24 '12 at 17:05
@BarryFandango - It's really not. Pay attention to the article. List is an IEnumerable – Justin Niessner Oct 24 '12 at 17:07
I do not really know why you were downvoted. Well explained :) – Picrofo Software Oct 24 '12 at 17:08
The downvote was based on Justin's original answer, which was just the first line. Not very helpful but got him to the top of the queue. It's getting more fleshed out as I watch. – James Orr Oct 24 '12 at 17:10
Or use `IReadOnlyList` (since .NET 4.5) as I commented in the other answer. – Jeppe Stig Nielsen Dec 05 '13 at 11:18

score 3 · Accepted Answer · edited May 23 '17 at 11:48

3

Interfaces can be variant; classes cannot. See here for the explanation.

Your code will work if you pass the strings collection uncasted and change the declaration to be

static void INeedToPassThisMethodAListOfObjects(IEnumerable<object> objects) { }

However, that depends on whether you need a List within this function.

edited May 23 '17 at 11:48

Community

1
1

answered Oct 24 '12 at 17:08

Tim Rogers

21,297
6
52
68

Thanks to everybody, Tim the prize goes to you for being the first to note about class type variance versus interfaces. – James Orr Oct 24 '12 at 17:17
Didn't I note this first? I don't care about points though so leave it like that :) – Pluc Oct 24 '12 at 19:34
Ah Pluc you're absolutely right, you had Tim Rogers beat by 90 seconds now that I look. Thanks for helping me. – James Orr Oct 25 '12 at 16:17
1

If @BarryFandango needs list functionality, in .NET 4.5 and later he can use `void Method(IReadOnlyList – Jeppe Stig Nielsen Dec 05 '13 at 11:16

score 2 · Answer 3 · edited Jun 20 '20 at 09:12

2

You have to convert the strings to objects. Theres a nice LINQ method to do so.

var strings = new List<string>();               
INeedToPassThisMethodAListOfObjects(strings.Cast<object>());

Edit

According to your own link,

In C#, variance is supported in the following scenarios:

Covariance in arrays (since C# 1.0)

Covariance and contravariance in delegates, also known as “method group variance” (since C# 2.0)

Variance for generic type parameters in interfaces and delegates (since C# 4.0)

So IEnumerable would accept List

edited Jun 20 '20 at 09:12

Community

1
1

answered Oct 24 '12 at 17:06

Pluc

2,909
1
22
36

`strings.Cast – Tim Rogers Oct 24 '12 at 17:10

Covariance/Conversion issue with Generics

3 Answers3