Why covariance does not work with generic method

问题

Assume I have interface and class:

public interface ITree {}
public class Tree : ITree {}

As IEnumerable<T> is covariant, the code line below is compiled successfully:

IEnumerable<ITree> trees = new List<Tree>();

But when I put it into generic method:

public void Do<T>() where T : ITree
{
     IEnumerable<ITree> trees = new List<T>();
}

I get compiled error from compiler:

Error 1 Cannot implicitly convert type 'System.Collections.Generic.List' to 'System.Collections.Generic.IEnumerable'. An explicit conversion exists (are you missing a cast?) D:\lab\Lab.General\Lab.General\Program.cs 83 40 Lab.General

Why covariance does not work in this case?

回答1:

That is because variance only works with reference types (classes, interfaces & delegates). Add a class constraint and it compiles just fine:

public static void Do<T>() where T : class, ITree

来源：https://stackoverflow.com/questions/12743444/why-covariance-does-not-work-with-generic-method