Microsoft KB Archive/197957

Knowledge Base

PRB: Passing Parameters By Reference to a VC COM Object

Article ID: 197957

Article Last Modified on 3/21/2005

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APPLIES TO

• Microsoft Active Server Pages 4.0
• Microsoft Visual C++ 5.0 Enterprise Edition
• Microsoft Visual C++ 6.0 Enterprise Edition
• Microsoft Visual C++ 5.0 Professional Edition
• Microsoft Visual C++ 6.0 Professional Edition
• Microsoft Visual C++ 6.0 Standard Edition
• Microsoft Internet Information Server 4.0
• Microsoft Internet Information Services 5.0

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This article was previously published under Q197957

SYMPTOMS

When passing a parameter to a COM Component's method in Active Server Pages (ASP) using VBScript, either the following error occurs:

Microsoft VBScript runtime error '800a000d' Type mismatch

Or the value stored in the variable passed to the method is not changed.

CAUSE

VBScript will pass parameters to a method by value if the argument's data type is NOT declared as a variant and the parameter is passed by reference if the argument's data type is declared as variant by the method.

RESOLUTION

Parameters to be passed as [out] parameters (that is, by reference) to a method should always be declared as a pointer to a variant data type by the method, while [in] parameters (that is, passed by value) can be declared as any type.

STATUS

This behavior is by design.

MORE INFORMATION

Steps to Reproduce Behavior

1. Create an ATL DLL Project called "ByRefProj."
2. Insert an ATL Object named "ByRefObj."
3. Add and implement the method.

   1. Add a method with the following information:

            Method Name: ByRefMethod
            Parameters : [out]BSTR* bstrVal
        
                              

   2. Implement the method as follows:

      STDMETHODIMP CByRefObj::ByRefMethod( BSTR* bstrVal )
      {
        CComBSTR bstrRtnVal = L"This variable is passed by Reference";
        *bstrVal = bstrRtnVal.Detach();
        return S_OK;
      }
                              

4. Create an Active Server Pages (ASP) page that calls this method:

   <%
     Dim objTest, strByRefVal
     Set objTest = Server.CreateObject("ByRefProj.ByRefObj")
     objTest.ByRefMethod strByRefVal
   %>

5. Run ASP, and the following error occurs:

   Microsoft VBScript runtime error '800a000d'
   Type mismatch: 'ByRefMethod'

To fix this problem, change the type to VARIANT:

1. Change the IDL declaration in the ByRefProj.idl file to:

   [id(1), helpstring("method ByRefMethod")] HRESULT ByRefMethod([out] VARIANT* vVal); 
                           

2. Change the method in the .h and .cpp file. Change the method as follows:

   // Where m_bstr is a BSTR member of SomeComObject.
   vVal->vt = VT_BSTR;
   vVal->bstrVal = m_bstr.Copy();
                           

3. Recompile.

REFERENCES

For additional information, click the article number below to view the article in the Microsoft Knowledge Base:

244012 INFO: Type Mismatch Errors When You Pass Parameters from ASP to a Visual Basic Component

Keywords: kberrmsg kbcodesnippet kbprb KB197957

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