How to solve: Conversion operators cannot convert from an interface type

Question

I've wrapped the INetFwRule members, I've put the properties in a custom type named FirewallRule, and for elegancy to save time in other parts of the code when performing conversions from INetFwRule to FirewallRule I tried to write this implicit converter:

C#:

public static explicit operator FirewallRule(INetFwRule rule) {
    return new FirewallRule {
        Action = (FirewallRuleAction)rule.Action,
        ApplicationName = rule.ApplicationName,
        Description = rule.Description,
        // etc...
    };
}

Vb.Net (original):

Public Shared Narrowing Operator CType(ByVal rule As INetFwRule) As FirewallRule

    Return New FirewallRule With {
        .Action = DirectCast(rule.Action, FirewallRuleAction),
        .ApplicationName = rule.ApplicationName,
        .Description = rule.Description,
        ' etc ...
    }

End Operator

However, I get this error in the rule parameter:

Conversion operators cannot convert from an interface type

There is an approach to solve this to be able write the CType?.

Answer 1

Compiler prohibits you from defining conversion operators that take interfaces as parameters, because it considers such conversion already defined (i.e. the built-in ability to cast).

You would need to use some other syntax to initiate conversions of INetFwRule to FirewallRule - for example, an extension to INetFwRule:

public static class NetFwRuleExt {
    public static FirewallRule ToFirewallRule(this INetFwRule rule) {
        return new FirewallRule {
            Action = (FirewallRuleAction)rule.Action,
            ApplicationName = rule.ApplicationName,
            Description = rule.Description,
            // etc...
        };
    }
}

Instead of writing

FirewallRule newRule = (FirewallRule)someRule;

clients of your API would write

FirewallRule newRule = someRule.ToFirewallRule();