C# Explicit Operators and Inheritance

Question

I'm sure this is a stupid question, but why does the following code not call the explicit operator for the cast on the child class MyBool?

public class DataType
{
    public static explicit operator bool(DataType D)
    {
        return false;
    }

    public static explicit operator DataType(bool B)
    {
        return new DataType();
    }
}

public class MyBool : DataType
{
    public bool Value;

    public MyBool()
    {
        Value = false;
    }

    public static explicit operator bool(MyBool B)
    {
        return B.Value;
    }

    public static explicit operator MyBool(bool B)
    {
        return new MyBool() { Value = B };
    }
}

then:

List<DataType> Types = new List<DataType>();

Types.Add(new MyBool() { Value = true });
Types.Add(new MyBool() { Value = false });

foreach (DataType T in Types)
{
    bool Value = (bool)T;
    MessageBox.Show(Value.ToString());
}

Produces the output: false, false

Is my only option to write functions on each class to take the place of the explicit operator functions?

My question was "why does the following code not call the explicit operator...?" not "is this working properly?". — Andy
– Andy, Commented Jun 7, 2011 at 9:53

Konrad Rudolph · Accepted Answer · 2011-06-06 12:39:10Z

21

why does the following code not call the explicit operator for the cast on the child class MyBool?

Because the operator functions are static, hence also non-virtual and thus their target is resolved at compile time rather than runtime. This is the expected behaviour.

If you want to have polymorphic conversion operators you can call virtual functions inside the operators:

public abstract class DataType
{
    public static explicit operator bool(DataType D)
    {
        return D.DoCastToBool();
    }

    public static explicit operator DataType(bool B)
    {
        // We haven’t got an instance of our class here.
        // You can use a factory method pattern to emulate virtual constructors.
    }

    protected abstract bool DoCastToBool();
}

edited Jun 6, 2011 at 12:39

answered Jun 6, 2011 at 12:09

Konrad Rudolph

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Comments

Jon Skeet · Accepted Answer · 2011-06-06 12:10:17Z

2

Operators are overloaded rather than overridden - in other words, the choice about which implementation to use is made at compile-time. The compiler only knows about T as DataType, so it calls the operator in DataType.

One option would be to remove the operator from MyBool, but add a virtual method in DataType, allowing for polymorphic behaviour:

public class DataType
{
    public static explicit operator bool(DataType D)
    {
        // TODO: Decide how you want to handle null references
        return D.ToBoolean();
    }

    protected virtual bool ToBoolean()
    {
        return false;
    }
}

public class MyBool : DataType
{
    // ...

    protected override bool ToBoolean()
    {
        return Value;
    }
}

Note that this won't work for the conversion from bool to a DataType, as in that case we don't have any information about which subtype of DataType you actually want to create.

(Side-note: your code would be easier to follow if you used the normal .NET naming conventions.)

answered Jun 6, 2011 at 12:10

Jon Skeet

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2 Comments

Sprague Over a year ago

Jon, has this always been the case in .NET versions? Or has there been a time when .NET searched inheritance hierarchies? I ask because a bug related to this seems to coincide with a project upgrade to newer .NET. The object in question is typed as 'object' at compile time, but the run-time type does have the operator defined. I highly doubt this is the case but it has been suggested...

Jon Skeet Over a year ago

@Sprague: No, it's always been the case.

Artjom B. · Accepted Answer · 2018-10-22 18:46:35Z

-1

Here's a garbage solution for you:

replace: bool Value = (bool)T;

with: bool Value = (bool)(T as MyBool);

edited Oct 22, 2018 at 18:46

Artjom B.

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answered Oct 22, 2018 at 18:17

Steven Ventura

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