Question

I’m generating output for a .Net executable from my own language… the opcode (called ‘Random’) that’s being translated from my language is supposed to create a random number within a specific range.

The goal for my code is to produce the random number using System.Reflection.Emit.ILGenerator class… to understand how the CIL code looks I’ve created some vb.net code:

Sub Main()
    Dim A As Random

    A = New Random

    Console.WriteLine(A.Next(100))
End Sub

Which ILDASM reports as:

.method public static void  Main() cil managed
{
  .entrypoint
  .custom instance void [mscorlib]System.STAThreadAttribute::.ctor() = ( 01 00 00 00 ) 
  // Code size       23 (0x17)
  .maxstack  2
  .locals init ([0] class [mscorlib]System.Random A)
  IL_0000:  nop
  IL_0001:  newobj     instance void [mscorlib]System.Random::.ctor()
  IL_0006:  stloc.0
  IL_0007:  ldloc.0
  IL_0008:  ldc.i4.s   100
  IL_000a:  callvirt   instance int32 [mscorlib]System.Random::Next(int32)
  IL_000f:  call       void [mscorlib]System.Console::WriteLine(int32)
  IL_0014:  nop
  IL_0015:  nop
  IL_0016:  ret
} // end of method Main::Main

I can reproduce everything using the ILGenerator.Emit method; except the line IL_0001 (“newobj instance void [mscorlib]System.Random::.ctor()”)…

Hopefully I haven’t overwhelmed anyone with too much information. But I figure it’s better to be verbose when describing a problem that seems complex to me.

Finally I have the code that I’ve produced so far:

 Sub EmitRandom()
    Dim NewRandom As New Random
    Dim stringtype As Type = GetType(System.Random)

    Dim paramtypes() As Type = {GetType(Integer)}, blankparams() As Type = {}
    'Dim RandomMethod = stringtype.GetMethod("New", paramtypes)

    m_ILGen.Emit(OpCodes.Newobj, New Random().GetType)

    EmitStoreInLocal(tempVariableRnd)
    EmitLoadLocal(tempVariableRnd)

    m_ILGen.Emit(OpCodes.Callvirt, stringtype.GetMethod("Next", paramtypes))
End Sub

Which emits the following code:

.
.
.
IL_0073:  newobj     [mscorlib]System.Random
IL_0078:  stloc.2
IL_0079:  ldloc.2
IL_007a:  callvirt   instance int32 [mscorlib]System.Random::Next(int32)
.
.
.

Things that I’ve tried already:

• Coming up with a way to point IL_Gen.Emit(OpCodes.NewObj, … ctor())… can’t figure out how.

• Coming up with a way to point to New() – since that seems to be what .ctor() is… New can’t be used as anything but an initializer.

• Just disabling the Random function until I can come up with a better way of emitting.

The problem seems hard to me but I know there’s someone out there who understands code generation and MSIL more easily than I do and who is willing to point out an answer.

Thanks for your time,

Dominick

Answer 1

You need to use the ConstructorInfo:

 m_ILGen.Emit(OpCodes.Newobj, GetType(Random).GetConstructor(Type.EmptyTypes))

Also – the storing and loading from local are unnecessary. You really just want the equivalent of new Random().Next(100), right?…in which case storing and loading from a local never happens.