Why does this explicit cast not work when there is an implicit conversion defined?

Question

I have a class with a defined implicit conversion to int:

public class SpeakerId
{
   private int value;
   public SpeakerId(int value) 
   { 
     this.value = value;
   }

   //integer conversions
   public static implicit operator SpeakerId(int value) => new SpeakerId(value);
   public static implicit operator int(SpeakerId obj) => obj.value;

   ...
}

With those conversions operators, I would think that this code would work:

var value = new SpeakerId(3);
var speakerId = (int)value;

but it doesn't; it causes a Specified cast is not valid error. Why would that be?

Update: My original code sample does work. The place where I get the error is when there's an intermediate cast to object, as you would have in a value converter:

public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
  var speakerId = (int)value;
            
  return ...
}

or, easier to see:

  var value = new SpeakerId(3);
  object intermediate = value;
  var speakerId = (int)intermediate;

Answer 1

So (int) against an object is unboxing, not casting, and doesn't invoke any conversion operators¹. You need to cast your object back to your reference type before you can try applying (int) to act as an actual cast:

var speakerId = (int)(SpeakerId)value;

Or you need to ensure you cast to int before it gets converted to object so that it is a boxed int and can be unboxed.

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Alternatively, if you don't know what the type of value is going to be but you're "sure" it'll have an appropriate conversion operator defined, you can force the lookup for a conversion to happen at runtime via dynamic:

var speakerId = (int)(dynamic)value;

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¹Except for some oddities around enums and their base types, unboxing has to be exactly to the type that was originally boxed.