Returning incorrect type from a function

Ok, I was scanning through some code yesterday and I saw this,

Code:

public class MyBaseClass
    {
        public string Field0()
        {
            return string.Empty;
        }
    }

    public class MyClass : MyBaseClass
    {
        public string Field1()
        {
            return String.Empty;
        }
    }

Pretty simple no complaints there. In the main code,

Code:

private void Form1_Load(object sender, EventArgs e)
        {

        }

        private MyBaseClass GetSomthing()
        {
            var retVal = new MyClass();
 开发者_运维技巧           return retVal;
        }

Now this code compiles correctly even though function GetSomthing() has a return type of MyBaseClass, it actually allows the return of MyClass(I assume that this is because of the inheritence).

I spotted this in somone elses code and to me it seems poor practice, although I may be missing somthing, anyone have any ideas?

Why is it poor practice? That's absolutely fine. For example, you might want:

public IEnumerable<string> GetNames()
{
    return new List<string> { "Fred", "Jon", "Joe" };        
}

It makes perfect sense from an encapsulation point of view - you're separating the interface of what you're willing to guarantee to the caller (it will be a sequence of strings) from the implementation (you're using a List<string>). This lets you change the implementation later without disturbing callers.

It's good practice rather than poor practice.

An instance of MyClass is also an instance of MyBaseClass, so it is perfectly valid (and common) to do that...