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?

Answer 1

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.