How to prevention when assignment a out param happening "Access Violation Exception"?

Question

There is method:

function Test.get_Param(out a : BOOL): HRESULT; stdcall;
begin
  a := b;
  Result := T_Result;
end;

Now the exception happening on a := b; , happening Access violation Exception. Ofcourse I can try and catch it. but I don't want to do that.... So Is there any way can determine use some way and skip the assignment. like:

if (! now I know it will happening that Exception){
    a := b; // so I can skip 
}
Result := T_Result;

Maybe it's very easy, but because I don't know use delphi, So hope your guys can help me. thanks.

Update1:

b: Boolean;//Some friend need to know what is the b param type.

Update2:

I'm try to use :

if b<> nil then Enabled := b;

but I can't build it , it will display: E2008 Incompatible types

Update3:

I'm trying to debug it, and when I'm debug, on the Local variables panel display:

a Inaccessible value

I'm use .NET called it. there is metadata:

bool get_Param{ [param: In, MarshalAs(UnmanagedType.Bool)] [PreserveSig] set; }

actually I'm not use .NET access it. I'm use .NET access a DirectShow filter, and the directshow filter is current method(write by delphi)

Update4:

this is partial C# code

[ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown), SuppressUnmanagedCodeSecurity, Guid("hidden")]
public interface IDCDSPFilterInterface{
    bool get_Param{ [param: In, MarshalAs(UnmanagedType.Bool)] [PreserveSig] set; } 
    ..            hidden other        ..
}}

Answer 1

There is a gross mismatch across the two sides of your interop boundary. Your Delphi function does not match the C# declaration.

The solution is not to test for parameter validity. Your Delphi code, given the declaration of the function in the question, is correct. The solution is to make both sides of the interop boundary match. I cannot tell you more than that until you show both sides of the interop boundary.