编译器如何确定是否可以将类型强制转换为其他类型

Question

Last week I handled some object casting (DataGridView Columns Control casts) and I tried to cast a DataGridView TextBox Column to TextBox Control where I had a compile time error.

I found out that I should cast that TextBox Column to DataGridViewTextBoxColumn.

So how does the compiler decide if a type can be cast to another type (objects mostly)?

Next you have cases where the compiler lets you to do some casts but you will get a run time error.

Answer 1

All classes and types inherit from object . Beyond that, it depends what class another class derives from. Using this, the compiler can determine if something can be cast from one to another. Some of these checks can be determined at compile-time, and some at run-time.

Consider:

class A
    class B
    class C
    class D
        class E

Now, give a reference to E , I can safely cast to D or A (or object ). The compiler can tell that if I try and cast E to C or B no conversion is available, because E does not inherit from these 2, though they share a common base-class of A .

Now, consider if I have a reference to A and attempt to cast to E . This check cannot be performed at compile time and will fail at runtime if the instance in question is not actually an instance of E (as, conceivably, it could be any of 'A, B, C, D or E').

In addition to this, as Silvermind points out, we can provide our own implicit and explicit conversions where none exist. Consider this:

public class Person
{
    private string name_;

    public Person(string name)
    {
        name_ = name;
    }

    // allow conversion to a string
    public static implicit operator string(Person p)
    {
        return p.name_;
    }
}

The above allows us to go:

Person p = new Person("Moo-Juice");
string name = (string)p;

Without our implicit operator, this would (obviously) fail at compile time as no conversion exists from Person to string by default.

Explicit operators are similar, but work the other way. This occurs if we wanted to cast a string to a person . So we could add the following explicit operator:

public static explicit operator Person(string name)
{
    return new Person(name);
}

And now we can do this:

string name = "Moo-Juice";
Person p = (Person)name;