[Mono-devel-list] Re: [mono-vb] VB Class status

Jambunathan Jambunathan kjambunathan at novell.com
Thu Jun 24 04:14:25 EDT 2004


> What makes that .param [n] appear there? 
> [opt]  comes from the Optional attribute? it seems so, but ...

( All these observations based on Windows XP box)
[opt] does get generated by [Optional]
.param does get generated by System.ComponentModel.DefaultValue.
Interestingly however, for the HasDefault flag to be turned on
the compiler apparently looks for ".param[] =  " construct.

Not sure what would segment of C# code would force
emitting of ".param[] = " construct. I believe that there is no such
construct in C# though.

Anyways the functionality is not broken ( if you use vbc as opposed to mbas ) 
only that the MSVB class status page simply refuse to recognize all the 
good effort that we have put in ...

Should corcompare be changed to accomodate this ? Shouldn't be a big deal
I think.

Jambunathan K.


Just out of curisoity I did this:
Compiled the following SampleClass.cs

using System;
using System.Runtime.InteropServices;
using  System.ComponentModel;
using System.Reflection;

class SampleClass
	public static void Main()
		Assembly thisAss = Assembly.GetExecutingAssembly();
		Type type = thisAss.GetType("SampleClass");
		MethodInfo method = type.GetMethod("SampleMethod");

		ParameterInfo [] parameters = method.GetParameters();
		foreach(ParameterInfo param in parameters)
			Console.WriteLine("param name = {0}, param attribute = {1}",
                                                                                param.Name, param.Attributes);
	public static void SampleMethod(int a, [Optional][DefaultValue(10)] int b)

I see that parameter b has Optional bit turned on but not the HasDefault bit.
The IL code for the above exe looks like this.

  .method public hidebysig static void  SampleMethod(int32 a,  [opt] int32 b) cil managed   
{     .param [2]  // notice that there is no "=something"  
      .custom instance void [System]System.ComponentModel.DefaultValueAttribute::.ctor(int32) = ( 01 00 0A 00 00 00 00 00 )      
      // Code size       7 (0x7)     .maxstack  1     
     IL_0000:  ldarg.1     
     IL_0001:  call       
     void [mscorlib]System.Console::WriteLine(int32)     
      IL_0006:  ret   
} // end of method SampleClass::SampleMethod

If in the above IL segment, I replace .param [2] with .param[2]=0x0000000A
and comment out .custom instance blah blah and compile the modified IL
back to an exe and execute the same I see that the parameter b has now
both Optional and HasDefault flags turned on.

>> "A Rafael D Teixeira" <rafaelteixeirabr at hotmail.com> 6/24/2004 1:13:54 AM >>>

>From: "Jambunathan Jambunathan" <kjambunathan at novell.com>
>The errors in Microsoft.VisualBasic may seem intimidating at first.
>Most of the errors are due to a mismatch in the Parameter Attributes.
>Many VB functions/methods support optional parameters with a default
>value. MS's dll marks the Parameter with Optional, HasDefault
>In our C# implementation of the method, we mark such parameters using
>following two attributes: System.Runtime.InteropServices.Optional
>and System.ComponentModel.DefaultValue(). As a result Mono's dll has
>Parameter Attributes just set to Optional ( but has no HasDeafult ).
>I am not sure whether this is bug or a baggage not carried from the
>( Quick googling suggested that just using C# it's not possible to
>create a parameter with Parameter Attributes set to Optional |
>Just posting to the mailing list with the hope that I will get a quick

It's indead trickier:

code like

   Private Sub x(ByVal w As String, Optional ByVal y As Integer = 132)
        TextBox1.Text = w & y
    End Sub

    Private Sub z()
        x("what", 1)
    End Sub

generates things like

.method private instance void  x(string w,
                                 [opt] int32 y) cil managed
  .param [2] = int32(0x00000084)
  .maxstack  8
  IL_0000:  nop
  IL_0001:  ldarg.0
  IL_001a:  ret
} // end of method Form1::x

we need to generate it properly for MS.VB.Dll but we still need more 
information to know how. What makes that .param [n] appear there? [opt] 
comes from the Optional attribute? it seems so, but ...

That is why I guessed DefaultValue, but it seems it is not the proper thing 
to use...

Maybe our truly Jackson of mono´s ilasm fame can bear some light for us...


.method private instance void  z() cil managed
  .maxstack  8
  IL_0000:  nop
  IL_0001:  ldarg.0
  IL_0002:  ldstr      "what"
  IL_0007:  ldc.i4.1
  IL_0008:  callvirt   instance void TesteXmlNText.Form1::x(string,
  IL_000d:  nop
  IL_000e:  ldarg.0
  IL_000f:  ldstr      "whatnot"
  IL_0014:  ldc.i4     0x84
  IL_0019:  callvirt   instance void TesteXmlNText.Form1::x(string,
  IL_001e:  nop
  IL_001f:  nop
  IL_0020:  ret
} // end of method Form1::z

that mbas must take care to generate, see that the compiler discover the 
default value and pass it
to the method, but it complicates even more when you add overload-resolution 
to the figure, and also late-binding (we will have to implement all that 
policies inside the helping classes in MS.VB.dll also).

So indeed a long way to go,

Have fun boys,

Rafael "Monoman" Teixeira
Mono Hacker since 16 Jul 2001 - http://www.go-mono.org/ 
MonoBrasil Founding Member - Membro Fundador do MonoBrasil 
English Blog: http://monoblog.blogspot.com/ 
Brazilian Portuguese Blog: http://monoblog.weblogger.terra.com.br/ 

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