What's going on with this method overloading?

Question

I have a question about method overloading in C#. I have a parent class and a child class.

class Parent
{
    public virtual string GetMyClassName()
    {
        return "I'm a Parent";
    }
}

class Child : Parent
{
    public override string GetMyClassName()
    {
        return "I'm a Child";
    }
}

I have two static methods declared outside of those two classes that act on objects of either type:

static string MyClassName(Parent parent)
{
    return "That's a Parent";
}

static string MyClassName(Child child)
{
    return "That's a Child";
}

When I test out how these methods are all called, I get what I think is a strange result:

Parent p = new Child();
var str1 = MyClassName(p); // = "That's a Parent"
var str2 = p.GetMyClassName(); // = "I'm a Child"

Why does str1 get set to "That's a Parent"? I am probably misunderstanding method overloading in C#. Is there a way to force the code use the Child call (setting str1 to "That's a Child")?

For reference: stackoverflow.com/questions/13095544/…

BradleyDotNET
– BradleyDotNET

2014-09-03 20:55:31 +00:00
Commented Sep 3, 2014 at 20:55 — BradleyDotNET
– BradleyDotNET, Commented Sep 3, 2014 at 20:55

Jon Skeet · Accepted Answer · 2014-09-03 20:53:34Z

7

Why does str1 get set to "That's a Parent"?

Because overloading is (usually) determined at compile-time, not execution time. It's based purely on the compile-time types of the target and arguments, with the exception of calls using dynamic values.

In your case, the argument type is Parent, so it calls MyClassName(Parent)

Is there a way to force the code use the Child call (setting str1 to "That's a Child")?

Two options:

Declare p as being of type Child, not Parent
Declare p as being of type dynamic, which will force the overload resolution to be performed at execution time

answered Sep 3, 2014 at 20:53

Jon Skeet

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2 Comments

Kyle W Over a year ago

Option 3: Call a single method and then check the type.

user2023861 Over a year ago

Wow, I've never come across a use for dynamic outside of Excel com interop calls before. Thanks.

cdhowie · Accepted Answer · 2014-09-03 20:53:33Z

2

Method overload resolution happens at compile time, while virtual method override resolution happens at run time.

Your call to MyClassName() is being resolved at compile time to the Parent overload because the type of p is Parent. Since the Child object is actually an instance of Parent too (due to inheritance) this is not a problem. (Note that p instanceof Parent is true, even if p references a Child object.)

answered Sep 3, 2014 at 20:53

cdhowie

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