InvalidCastException with COM set methods
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Thread: InvalidCastException with COM set methods

  1. #1
    Join Date
    Mar 2009
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    InvalidCastException with COM set methods

    Hi All,

    I am referring to an ActiveX dll in my windows project when i looked into metatdata there are some methods like say set_Test(ref object __MIDL_0543).
    Now i want to set Test from my .net code, in vb code it is set as
    Set objectName.Test = otherObject
    With c# it is done like
    object newObject = otherObject // otherObject is of type say Other
    objectName.set_Test(ref newObject)

    The above line throws then an InvalidCastException, so what should we do here.

    Please suggest.

    Thanks,
    Anurodh

  2. #2
    Join Date
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    Re: InvalidCastException with COM set methods

    Hi !
    At first please reread forum usage and use codetags as we are not interested in reading code-sausage But without using codetags you will get unformatted code.
    Code:
    object newObject = otherObject // otherObject is of type say Other
    objectName.set_Test(ref newObject)
    In C# you need to declare the exact class if you want to use the object. You need to know which class newObject is before you are able to use it So you need to have knowledge about the dll before you are able to use it. Thas simple all about it.
    Jonny Poet

    To be Alive is depending on the willingsness to help others and also to permit others to help you. So lets be alive. !
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  3. #3
    Join Date
    Mar 2009
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    Re: InvalidCastException with COM set methods

    Thanks for the reply.

    But it doesn't seems to be that simple. The real problem is the syntax generated by VS i.e. set_Test(ref object __MIDL_0543).

    we have checked in the vb code that runs fine that it takes an object say ITest and Test implements ITest.

    so, in c# we did it like:
    Test t1 = new Test();
    ITest t2 = t1;
    object t3 = t2;

    objectName.set_Test(ref t3); // throws invalidcastexception

    Please suggest any solution on this.

    Thanks,
    Anurodh
    Last edited by anurodhora; June 11th, 2009 at 01:55 AM.

  4. #4
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    Re: InvalidCastException with COM set methods

    Quote Originally Posted by anurodhora View Post
    so, in c# we did it like:
    Code:
    Test t1 = new Test();
    ITest t2 = t1;
    object t3 = t2; 'useless 
      
    objectName.set_Test(ref t3); // throws invalidcastexception
    and what is objectname declared to be ? Does it have a set_Test method ?
    There is nothing to suggest. this cannot be done that way in C# an object cannot ne used that way. Never, whatever it contains, you need to call it as the interface. Try
    Code:
    ITest t2 = new Test();// you can create a class and refer it directly by a pointer to its interface
    objectName.set_Test(ref t2);
    And please use CODETAGS
    Jonny Poet

    To be Alive is depending on the willingsness to help others and also to permit others to help you. So lets be alive. !
    Using Code Tags makes the difference: Code is easier to read, so its easier to help. Do it like this: [CODE] Put Your Code here [/code]
    If anyone felt he has got help, show it in rating the post.
    Also dont forget to set a post which is fully answered to 'resolved'. For more details look to FAQ's about Forum Usage. BTW I'm using Framework 3.5 and you ?
    My latest articles :
    Creating a Dockable Panel-Controlmanager Using C#, Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7

  5. #5
    Join Date
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    Posts
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    Re: InvalidCastException with COM set methods

    I think that the class of the object beeing set (Test in your case) must be at least annotated with [ComVisible(true)] (or the whole assembly) and maybe it should implement some interface, which should be mentioned in reference.
    • Make it run.
    • Make it right.
    • Make it fast.

    Don't hesitate to rate my post.

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