Dynamic... But Fast: The Tale of Three Monkeys, A Wolf and the DynamicMethod and ILGenerator Classes

Imports System.Data.SqlClient
Imports System.Reflection
Imports System.Reflection.Emit
Imports System.Data

Public Class DynamicBuilder(Of T)
    Private Shared ReadOnly getValueMethod As MethodInfo = GetType(IDataRecord).GetMethod("get_Item", New Type() {GetType(Integer)})
    Private Shared ReadOnly isDBNullMethod As MethodInfo = GetType(IDataRecord).GetMethod("IsDBNull", New Type() {GetType(Integer)})
    Private Delegate Function Load(ByVal dataRecord As IDataRecord) As T
    Private handler As Load

    Private Sub DynamicBuilder()

    End Sub

    Public Function Build(ByVal dataRecord As IDataRecord) As T
        Return handler(dataRecord)
    End Function


    Public Shared Function CreateBuilder(ByVal dataRecord As IDataRecord) As DynamicBuilder(Of T)
        Dim dynamicBuilder As DynamicBuilder(Of T) = New DynamicBuilder(Of T)()
        Dim i As Integer

        Dim method As DynamicMethod = New DynamicMethod("DynamicCreate", GetType(T), New Type() {GetType(IDataRecord)}, GetType(T), True)
        Dim generator As ILGenerator = method.GetILGenerator()

        Dim result As LocalBuilder = generator.DeclareLocal(GetType(T))
        generator.Emit(OpCodes.Newobj, GetType(T).GetConstructor(Type.EmptyTypes))
        generator.Emit(OpCodes.Stloc, result)

        For i = 0 To dataRecord.FieldCount - 1
            Dim propertyInfo As PropertyInfo = GetType(T).GetProperty(dataRecord.GetName(i))
            Dim endIfLabel As Label = generator.DefineLabel()

            If Not propertyInfo Is Nothing Then
                If Not propertyInfo.GetSetMethod() Is Nothing Then
                    generator.Emit(OpCodes.Ldarg_0)
                    generator.Emit(OpCodes.Ldc_I4, i)
                    generator.Emit(OpCodes.Callvirt, isDBNullMethod)
                    generator.Emit(OpCodes.Brtrue, endIfLabel)

                    generator.Emit(OpCodes.Ldloc, result)
                    generator.Emit(OpCodes.Ldarg_0)
                    generator.Emit(OpCodes.Ldc_I4, i)
                    generator.Emit(OpCodes.Callvirt, getValueMethod)
                    generator.Emit(OpCodes.Unbox_Any, dataRecord.GetFieldType(i))
                    generator.Emit(OpCodes.Callvirt, propertyInfo.GetSetMethod())

                    generator.MarkLabel(endIfLabel)
                End If
            End If
        Next

        generator.Emit(OpCodes.Ldloc, result)
        generator.Emit(OpCodes.Ret)

        dynamicBuilder.handler = CType(method.CreateDelegate(GetType(Load)), Load)
        Return dynamicBuilder
    End Function
End Class