How to use GetType and GetFields? - vb.net

Question

I'm updating a program and there is about 40 classes. I need to create a function that takes two lists of type object as parameters. Both lists only have one item in them (A version of the item BEFORE any changes were made, and AFTER the changes happened). I'm using these objects to create a single object to implement an UNDO button. With these parameters I need to get the type of each and make sure they match, if not then something went wrong. Next I'll need to read in the fields/properties/members (Not sure what to choose) and then compare them to each other and find what changed so I can set that as the item description. I don't want to trace the whole code and add specific functions for each and I know there has got to be a way to do this generically. I have created this small mock-up program that semi works for what I am trying to do. I can get the class type out of the object in list, but i have no idea how to get fields or whatever.
I'm using a large database with entity framework.
Also Using VB.NET!
Thanks for the help!
Here's code for generic program:
Imports System.Reflection
Module Module1
Sub Main()
Dim Myself As New Human("Matthew", "Cucco", Now, "Blonde", 19, False)
Dim NotMe As New Human("Jake", "Cucco", Now, "Blonde", 19, False)
Dim Him As New Employee("Matt", "Cucco", Now, "Blonde", 19, False, 215, "LuK", True)
Dim Her As New Customer("Jessie", "Keller", Now, "Blonde", 19, True, 25, "Cereal", "me#gmail.com")
Dim ListofPeople As IList(Of Object) = {Myself, NotMe, Him, Her}
Dim ListofPeople2 As IList(Of Object) = {Myself, NotMe, Him, Her}
ObjectsAreSameClass(ListofPeople, ListofPeople2)
Console.ReadKey()
End Sub
Private Function ObjectsAreSameClass(object1 As IList(Of Object), object2 As IList(Of Object)) As Boolean
Dim ObjectType As Type = object1.First.GetType()
Dim AreSameClass As Boolean = Nothing
Console.WriteLine(ObjectType.ToString)
If (object1.First.GetType() = object2.First.GetType()) Then
AreSameClass = True
Console.WriteLine("Object1 is of type: " + object1.First.GetType().Name)
Console.WriteLine("Object2 is of type: " + object2.First.GetType().Name)
If (object1.First.GetType().Name = "Human") Then
Console.WriteLine("Yep this works")
End If
Else
AreSameClass = False
Console.WriteLine("Object1 is of type: " + object1.First.GetType().Name)
Console.WriteLine("Object2 is of type: " + object2.First.GetType().Name)
If (object1.First.GetType().Name = "Human") Then
Console.WriteLine("Yep this works")
Console.WriteLine(object1.First.GetType().GetFields().ToString)
End If
End If
Dim MyField As PropertyInfo() = ObjectType.GetProperties()
Dim i As Integer
For i = 0 To MyField.Length - 1
Console.WriteLine(MyField(i).ToString)
Next i
Console.WriteLine("Objects are equal? t/f : " + AreSameClass.ToString)
Return AreSameClass
End Function
Public Class Human
Public FirstName As String
Public LastName As String
Public Birthdate As Date
Public HairColor As String
Public Age As Integer
Public Gender As Boolean 'False for male, true for female
Public Sub New()
FirstName = ""
LastName = ""
Birthdate = Now
HairColor = ""
Age = 0
Gender = False
End Sub
Public Sub New(f As String, l As String, b As Date, h As String, a As Integer, g As Boolean)
FirstName = f
LastName = l
Birthdate = b
HairColor = h
Age = a
Gender = g
End Sub
End Class
Public Class Employee
Inherits Human
Dim EmployeeId As Integer
Dim PlaceOfEmployment As String
Dim IsManager As Boolean
Public Sub New()
MyBase.New()
EmployeeId = 0
PlaceOfEmployment = ""
IsManager = False
End Sub
Public Sub New(f As String, l As String, b As Date, h As String, a As Integer, g As Boolean, i As Integer, p As String, m As Boolean)
MyBase.New(f, l, b, h, a, g)
EmployeeId = i
PlaceOfEmployment = p
IsManager = m
End Sub
End Class
Public Class Customer
Inherits Human
'used for testing
Dim IdNumber As Integer
Dim FavoriteItem As String
Dim email As String
Public Sub New()
MyBase.New()
IdNumber = 0
FavoriteItem = ""
email = ""
End Sub
Public Sub New(f As String, l As String, b As Date, h As String, a As Integer, g As Boolean, i As Integer, fav As String, e As String)
MyBase.New(f, l, b, h, a, g)
IdNumber = i
FavoriteItem = fav
email = e
End Sub
End Class
End Module
This Currently displays this:
TestProject.Module1+Human
Object1 is of type: Human
Object2 is of type: Human
Yep this works
Objects are equal? t/f : True
Also for reference, here is my main program that I will be implementing this into:
Function NewItem(Before As IEnumerable(Of Object), After As IEnumerable(Of Object), ObjectType As String)
ObjectsAreSameClass(Before, After, ObjectType) 'Check if objects are same class
Dim BeforeFields() As FieldInfo = GetFieldData(Before) 'gets all field info, saves to an array
Dim AfterFields() As FieldInfo = GetFieldData(After)
'Now check and make sure the objects are not the same
Dim ThisChanged As FieldInfo
If (ObjectValuesAreEqual(BeforeFields, AfterFields) = True) Then
'These objects did not not change
ThisChanged = Nothing
Else
'Change occured, find out where
ThisChanged = FindWhatChanged(BeforeFields, AfterFields)
End If
'Create a new UndoRedo item and give it these values
Dim UndoRedoNow As New ClsUndoRedo
UndoRedoNow.BeforeObject = Before.Single
UndoRedoNow.AfterObject = After.Single
UndoRedoNow.ObjectCounter += 1
UndoRedoNow.WhatChanged = ThisChanged
If WhatGroupChanged.isDeleted Then
UndoRedoNow.WhatAction = Before.Single.GetType().ToString + " item was Deleted"
ElseIf WhatGroupChanged.isNew Then
UndoRedoNow.WhatAction = After.Single.GetType().ToString + " item was created"
ElseIf WhatGroupChanged.isChanged Then
UndoRedoNow.WhatAction = After.Single.GetType().ToString + " item was changed"
End If
UndoRedoNow.WhatGroupChanged.isRedo = False 'Make sure it is not a redo object
'Now add object to list
ChangeLog.Add(UndoRedoNow)
Return Nothing
End Function
Private Function ObjectsAreSameClass(before As IEnumerable(Of Object), after As IEnumerable(Of Object), WhatType As String) As Boolean
Dim AreSameClass As Boolean = False
Try
If (before.Single.GetType() = after.Single.GetType() Or (before Is Nothing) Or (after Is Nothing)) Then
'Objects are of the same class or nothing
If before Is Nothing Then
WhatGroupChanged.isNew = True 'New item
ElseIf after Is Nothing Then
WhatGroupChanged.isDeleted = True 'Deleted item
Else
WhatGroupChanged.isChanged = True 'item was changed
End If
AreSameClass = True
End If
Catch
'Need to raise error
End Try
Return AreSameClass
End Function
''' <summary>
''' This function will return all of the fields for a certain class as well as the data stored in them
''' </summary>
''' <param name="list"></param>
''' <returns></returns>
Public Shared Function GetFieldData(ByVal list As IList(Of Object)) As FieldInfo()
Dim fields() As FieldInfo = list.Single.GetType().GetFields()
Return fields
End Function
''' <summary>
''' This function will check that the values in the datafields are not equal
''' </summary>
''' <param name="Before"></param>
''' <param name="After"></param>
''' <returns></returns>
Private Function ObjectValuesAreEqual(Before() As FieldInfo, After() As FieldInfo) As Boolean
Dim isEqual As Boolean = New Boolean 'This will keep track of if the elements are equal or not
For index As Integer = 0 To (Before.Count - 1)
If Before.ElementAt(index).GetValue(Before.ElementAt(index)).Equals(After.ElementAt(index).GetValue(After.ElementAt(index))) Then
'They are equal so set to true
isEqual = True
Else
'They are not equal so set to false and return
isEqual = False
Return isEqual
End If
Next
Return isEqual
End Function
Private Function FindWhatChanged(Before() As FieldInfo, After() As FieldInfo) As FieldInfo
Dim ThisIsChange As FieldInfo
For index As Integer = 0 To (Before.Count - 1)
If Before.ElementAt(index).GetValue(Before.ElementAt(index)).Equals(After.ElementAt(index).GetValue(After.ElementAt(index))) Then
ThisIsChange = After.ElementAt(index)
Return ThisIsChange
Else
'Raise error
End If
Next
End Function

Answer 1

The proper way to preserve type information when working with unknown types is to write a generic function (and if necessary generic classes, structures, etc.).
Using GetType, in a perfect world, should never be needed.
Generic functions look like this:
Public Function MyGenericFunction(Of T)(myArg as T) as Integer
' do something with myArg1, myArg2 ... without knowing their exact type
Return 0
End Function
' or with multiple types
Public Function MyGenericFunction2(Of T1, T2, ... )(myArg1 as T1, myArg2 as T2, ...) as T1()
' do something with myArg1, myArg2 ... without knowing their exact type
Return { myArg1 }
End Function
When you call these functions, the generic types are usually automatically deduced from the arguments you passed. If they can't be guessed, you will need to explicitly annotate the types, like this:
Dim x = MyGenericFunction(Of SomeClass1)(foo)
Dim x = MyGenericFunction(Of SomeClass2)(foo)
A full guide here: https://msdn.microsoft.com/en-us/library/w256ka79.aspx
However, if you need to handle specific types with the same function, then what you want to do use is a more narrow tool: overloading, or more technically parametric polymorphism.
What that means is, you will need to provide two (or more) different definitions of the same function ( = having the same name), that accept parameters of different types.
A simple example:
Public Class MyClass1
Public Foo1 As String = "foo1"
End Class
Public Class MyClass2
Public Foo2 As String = "foo2"
End Class
Public Sub MyFunction(arg as MyClass1)
Console.WriteLine(arg.Foo1)
End Sub
Public Sub MyFunction(arg as MyClass2)
Console.WriteLine(arg.Foo2)
End Sub
Dim x as Object
' let's give x a random value of either MyClass1 or MyClass2,
' and we don't know in advance which one
If DateTime.Today.DayOfWeek = DayOfWeek.Tuesday Then
x = new MyClass1
Else
x = new MyClass2
End If
' the program will automatically invoke the correct function based on x's value, and print either "foo1" or "foo2"
MyFunction(x)