Imports System.Reflection
Imports System.Reflection.Emit

' These classes are for demonstration purposes.
'
Public Class Base
   Public Number As Integer = 0

   Public Sub New(ByVal initialNumber As Integer)
      Number = initialNumber 
   End Sub
End Class

Public Class DerivedFromBase
   Inherits Base

   Public Sub New(ByVal initialNumber As Integer)
      MyBase.New(initialNumber)
   End Sub
End Class

' Two delegates are declared: UseLikeInstance treats the dynamic
' method as if it were an instance method, and UseLikeStatic
' treats the dynamic method in the ordinary fashion.
' 
Public Delegate Function UseLikeInstance(ByVal number As Integer) _
    As Integer
Public Delegate Function UseLikeStatic(ByVal b As Base, _
    ByVal number As Integer) As Integer

Public Class Example

   Public Shared Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      ' This dynamic method sets the public Number field of an instance
      ' of Base, and returns the previous value. The method has no name. 
      ' It takes two parameters, an instance of Base and an Integer that
      ' is the new field value. 
      '
      Dim changeNumber As New DynamicMethod( _
          "", _
          GetType(Integer), _
          New Type() {GetType(Base), GetType(Integer)} _
      )

      ' Get a FieldInfo for the public field 'Number'.
      Dim fnum As FieldInfo = GetType(Base).GetField( _
          "Number", _
          BindingFlags.Public Or BindingFlags.Instance _
      )

      Dim ilg As ILGenerator = changeNumber.GetILGenerator()

      ' Push the current value of the Number field onto the 
      ' evaluation stack. It's an instance field, so load the
      ' instance of Base before accessing the field.
      ilg.Emit(OpCodes.Ldarg_0)
      ilg.Emit(OpCodes.Ldfld, fnum)

      ' Load the instance of Base again, load the new value 
      ' of id, and store the new field value. 
      ilg.Emit(OpCodes.Ldarg_0)
      ilg.Emit(OpCodes.Ldarg_1)
      ilg.Emit(OpCodes.Stfld, fnum)

      ' The original value of the Number field is now the only 
      ' thing on the stack, so return from the call.
      ilg.Emit(OpCodes.Ret)


      ' Create a delegate that uses changeNumber in the ordinary
      ' way, as a static method that takes an instance of
      ' Base and an Integer.
      '
      Dim uls As UseLikeStatic = CType( _
          changeNumber.CreateDelegate(GetType(UseLikeStatic)),  _
          UseLikeStatic _
      )

      ' Create an instance of Base with an id of 42.
      '
      Dim ba As New Base(42)

      ' Create a delegate that is bound to the instance of 
      ' of Base. This is possible because the first 
      ' parameter of changeNumber is of type Base. The 
      ' delegate has all the parameters of changeNumber except
      ' the first.
      Dim uli As UseLikeInstance = CType( _
          changeNumber.CreateDelegate( _
              GetType(UseLikeInstance), _
              ba),  _
          UseLikeInstance _
      )

      ' First, change the value of Number by calling changeNumber as
      ' a static method, passing in the instance of Base.
      '
      outputBlock.Text &= String.Format( _
          "Change the value of Number; previous value: {0}", _
          uls(ba, 1492) _
      ) & vbCrLf

      ' Change the value of Number again using the delegate 
      ' bound to the instance of Base.
      '
      outputBlock.Text &= String.Format( _
          "Change the value of Number; previous value: {0}", _
          uli(2700) _
      ) & vbCrLf

      outputBlock.Text &= String.Format("Final value of Number: {0}", ba.Number) & vbCrLf


      ' Now repeat the process with a class that derives
      ' from Base.
      '
      Dim dfba As New DerivedFromBase(71)

      uli = CType( _
          changeNumber.CreateDelegate( _
              GetType(UseLikeInstance), _
              dfba),  _
          UseLikeInstance _
      )

      outputBlock.Text &= String.Format( _
          "Change the value of Number; previous value: {0}", _
          uls(dfba, 73) _
      ) & vbCrLf
      outputBlock.Text &= String.Format( _
          "Change the value of Number; previous value: {0}", _
          uli(79) _
      ) & vbCrLf
      outputBlock.Text &= String.Format("Final value of Number: {0}", dfba.Number) & vbCrLf

   End Sub
End Class

' This example produces the following output:
'
'Change the value of Number; previous value: 42
'Change the value of Number; previous value: 1492
'Final value of Number: 2700
'Change the value of Number; previous value: 71
'Change the value of Number; previous value: 73
'Final value of Number: 79'