{"id":696,"date":"2013-10-02T18:12:49","date_gmt":"2013-10-02T14:12:49","guid":{"rendered":"http:\/\/prgmr.io\/?p=696"},"modified":"2013-10-02T18:21:39","modified_gmt":"2013-10-02T14:21:39","slug":"what-is-reflection","status":"publish","type":"post","link":"https:\/\/rakhesh.com\/powershell\/what-is-reflection\/","title":{"rendered":"What is Reflection?"},"content":{"rendered":"

I knew that PowerShell constructs such [bool]<\/code> – which defines the boolean data type – were called accelerators, but I never knew why and didn’t bother much as I figured I’d learn more about them soon or later. <\/p>\n

But yesterday while reading Bruce Payette’s “Windows PowerShell in Action” that book too mentioned the word accelerator without defining it, and so I started Googling a bit about it. This Googling introduced me to accelerators for WMI – very useful, something I must explore sometime – and that got me curious on how to find all the available accelerators in PowerShell. This in turn bought me to the topic of reflection as that’s what one uses to get a list of accelerators in PowerShell. <\/p>\n

So what is reflection? Truth to be told it still does not make much sense to be but what I understand is that reflection is the act (or ability) of an object to look at itself (see it’s “reflection” in the mirror so to say) and learn what it’s capable of – what methods it has, what are it’s properties, and so on. And it is useful because sometimes you may encounter objects that you are getting from elsewhere, and reflection is what you need to learn more about the object. <\/p>\n

I guess an example will make it clearer. <\/p>\n

Say I define an object called $date<\/code> as below:<\/p>\n

$date = [datetime]\"13 March 2013\"<\/pre>\n
As the creator of this object I know it’s of type [datetime]<\/code>. And since PowerShell provides a cmdlet Get-Members<\/code> I know I can use it to examine the methods and properties of this object. <\/p>\n
But what if I wasn’t the creator of this object. All I know is that I have a script which will get some object as input and I am supposed to examine it and take further action depending on what sort of an object it is. Further, assume the Get-Member<\/code> cmdlet isn’t available, or for some reason you can’t use it on the object you are receiving (or maybe Get-Member<\/code> itself depends on reflection, which we’ll talk about below, so it won’t work unless the concept of reflection is supported by PowerShell). Either ways – you don’t have Get-Member<\/code> available, period. <\/p>\n
In such a situation how would you know what is contained inside the object? That’s where reflection comes into the picture. <\/p>\n
Before we go further, it’s worth reading this<\/a> WikiPedia page on reflection, and this<\/a> and this<\/a> StackOverflow post. All these links explain reflection, but the takeaway from the Wikipedia page are the examples it gives of reflection in various languages, and the takeaway from one of the answers<\/a> in the second StackOverflow post is that all objects in Java have a getClass<\/code> method which lets one reflectively examine an unknown object. <\/p>\n
I created a custom PowerShell object to see if PowerShell objects have a similar method. <\/p>\n
\r\nPS> $blah = New-Object -TypeName PSObject -Property @{ Name = \"Rakhesh\"; Age = 34 }\r\nPS> $blah | Get-Member\r\n\r\n\r\n   TypeName: System.Management.Automation.PSCustomObject\r\n\r\nName        MemberType   Definition\r\n----        ----------   ----------\r\nEquals      Method       bool Equals(System.Object obj)\r\nGetHashCode Method       int GetHashCode()\r\nGetType     Method       type GetType()\r\nToString    Method       string ToString()\r\nAge         NoteProperty System.Int32 Age=34\r\nName        NoteProperty System.String Name=Rakhesh\r\n<\/pre>\n
Sure enough, GetType()<\/code> looks like what we are after. (GetHashcode()<\/code> and ToString<\/code> return an integer and string respectively (as seen from their definitions) so we can straightaway eliminate them). <\/p>\n
Let’s dig deeper:<\/p>\n
\r\nPS> $blah.GetType()\r\n\r\nIsPublic IsSerial Name                                     BaseType\r\n-------- -------- ----                                     --------\r\nTrue     False    PSCustomObject                           System.Object\r\n\r\nPS> get-member -InputObject $blah.GetType()\r\n\r\n\r\n   TypeName: System.RuntimeType\r\n\r\nName                           MemberType Definition\r\n----                           ---------- ----------\r\nAsType                         Method     type AsType()\r\nClone                          Method     System.Object Clone(), System.Object ICloneable.Clone()\r\nEquals                         Method     bool Equals(System.Object obj), bool Equals(type o), bool _Membe...\r\n...\r\nGetMember                      Method     System.Reflection.MemberInfo[] GetMember(string name, System.Ref...\r\nGetMembers                     Method     System.Reflection.MemberInfo[] GetMembers(System.Reflection.Bind...\r\nGetMethod                      Method     System.Reflection.MethodInfo GetMethod(string name, System.Refle...\r\nGetMethods                     Method     System.Reflection.MethodInfo[] GetMethods(System.Reflection.Bind...\r\nGetNestedType                  Method     type GetNestedType(string fullname, System.Reflection.BindingFla...\r\nGetNestedTypes                 Method     type[] GetNestedTypes(System.Reflection.BindingFlags bindingAttr...\r\nGetObjectData                  Method     void GetObjectData(System.Runtime.Serialization.SerializationInf...\r\nGetProperties                  Method     System.Reflection.PropertyInfo[] GetProperties(System.Reflection...\r\nGetProperty                    Method     System.Reflection.PropertyInfo GetProperty(string name, System.R...\r\nGetType                        Method     type GetType(), type _MemberInfo.GetType(), type _Type.GetType()\r\n...\r\nName                           Property   string Name {get;}\r\nNamespace                      Property   string Namespace {get;}\r\nReflectedType                  Property   type ReflectedType {get;}\r\nStructLayoutAttribute          Property   System.Runtime.InteropServices.StructLayoutAttribute StructLayou...\r\nTypeHandle                     Property   System.RuntimeTypeHandle TypeHandle {get;}\r\nTypeInitializer                Property   System.Reflection.ConstructorInfo TypeInitializer {get;}\r\nUnderlyingSystemType           Property   type UnderlyingSystemType {get;}\r\n<\/pre>\n
The GetType()<\/code> method returns an object of type RuntimeType<\/code> which contains many methods and properties. The interesting bits for us are GetProperties<\/code>, GetMethods<\/code>, and so on. Let’s explore these:<\/p>\n
\r\nPS> $blah.GetType().GetMethods()\r\n\r\n\r\nName                       : ToString\r\nDeclaringType              : System.Management.Automation.PSCustomObject\r\nReflectedType              : System.Management.Automation.PSCustomObject\r\nMemberType                 : Method\r\nMetadataToken              : 100667351\r\nModule                     : System.Management.Automation.dll\r\nIsSecurityCritical         : True\r\nIsSecuritySafeCritical     : True\r\nIsSecurityTransparent      : False\r\nMethodHandle               : System.RuntimeMethodHandle\r\nAttributes                 : PrivateScope, Public, Virtual, HideBySig\r\nCallingConvention          : Standard, HasThis\r\nReturnType                 : System.String\r\nReturnTypeCustomAttributes : System.String\r\nReturnParameter            : System.String\r\nIsGenericMethod            : False\r\nIsGenericMethodDefinition  : False\r\nContainsGenericParameters  : False\r\nMethodImplementationFlags  : IL\r\nIsPublic                   : True\r\nIsPrivate                  : False\r\nIsFamily                   : False\r\nIsAssembly                 : False\r\nIsFamilyAndAssembly        : False\r\nIsFamilyOrAssembly         : False\r\nIsStatic                   : False\r\nIsFinal                    : False\r\nIsVirtual                  : True\r\nIsHideBySig                : True\r\nIsAbstract                 : False\r\nIsSpecialName              : False\r\nIsConstructor              : False\r\nCustomAttributes           : {}\r\n\r\n# snipping out similar output for each of the other methods\r\n\r\nPS> $blah.GetType().GetProperties()\r\n# no result here as this is a custom PSObject and it doesn't \r\n# have properties, only NoteProperties. see get-member output above. \r\n<\/pre>\n
Now let’s try this on our “unknown” $date<\/code> object:<\/p>\n
\r\nPS> $date.GetType()\r\n\r\nIsPublic IsSerial Name                                     BaseType\r\n-------- -------- ----                                     --------\r\nTrue     True     DateTime                                 System.ValueType\r\n\r\n# find the class name of the object\r\nPS> $date.GetType().Name\r\nDateTime\r\n\r\n# get its members, format the name field of the output as columns as there's a lot\r\nPS> $date.GetType().GetMethods() | fw Name -Column 4\r\n\r\n\r\nget_Date                    get_Day                     get_DayOfWeek              get_DayOfYear\r\nget_Hour                    get_Kind                    get_Millisecond            get_Minute\r\nget_Month                   get_Now                     get_UtcNow                 get_Second\r\nget_Ticks                   get_TimeOfDay               get_Today                  get_Year\r\nAdd                         AddDays                     AddHours                   AddMilliseconds\r\nAddMinutes                  AddMonths                   AddSeconds                 AddTicks\r\nAddYears                    Compare                     CompareTo                  CompareTo\r\nDaysInMonth                 Equals                      Equals                     Equals\r\nFromBinary                  FromFileTime                FromFileTimeUtc            FromOADate\r\nIsDaylightSavingTime        SpecifyKind                 ToBinary                   GetHashCode\r\n...\r\n\r\n# how can I invoke a method of the object? \r\n# the output above is an array, I want to invoke the AddYears() method - that's at location 24 - so let's try:\r\nPS> $date.GetType().GetMethods()[24]\r\n\r\n\r\nName                       : AddYears\r\nDeclaringType              : System.DateTime\r\nReflectedType              : System.DateTime\r\nMemberType                 : Method\r\nMetadataToken              : 100664410\r\n...\r\n\r\n# this gives the details of the method. but how do I invoke it?\r\n# I could use Get-Member here to examine the object above, but we don't have Get-Member remember?\r\n# so I'll use reflection again.\r\nPS> $date.GetType().GetMethods()[24].GetType().GetMethods() | fw Name -Column 4\r\n\r\nget_Name                    get_DeclaringType           get_ReflectedType          get_MemberType\r\nget_MetadataToken           get_Module                  get_IsSecurityCritical     get_IsSecuritySafeCritical\r\nget_IsSecurityTransparent   get_MethodHandle            get_Attributes             get_CallingConvention\r\nget_ReturnType              get_ReturnTypeCustomAttr... get_ReturnParameter        get_IsGenericMethod\r\nget_IsGenericMethodDefin... get_ContainsGenericParam... get_MethodImplementatio... get_IsPublic\r\nget_IsPrivate               get_IsFamily                get_IsAssembly             get_IsFamilyAndAssembly\r\nget_IsFamilyOrAssembly      get_IsStatic                get_IsFinal                get_IsVirtual\r\nget_IsHideBySig             get_IsAbstract              get_IsSpecialName          get_IsConstructor\r\nget_CustomAttributes        ToString                    GetHashCode                Equals\r\nGetCustomAttributes         GetCustomAttributes         IsDefined                  GetCustomAttributesData\r\nGetParameters               GetMethodImplementationF... GetMethodBody              Invoke\r\nGetBaseDefinition           CreateDelegate              CreateDelegate             MakeGenericMethod\r\nGetGenericArguments         GetGenericMethodDefinition  GetObjectData              Invoke\r\nGetType\r\n\r\n# the Invoke method seems useful, so let's try that\r\nPS> $date.GetType().GetMethods()[24].Invoke\r\n\r\nOverloadDefinitions\r\n-------------------\r\nSystem.Object Invoke(System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder\r\nbinder, System.Object[] parameters, cultureinfo culture)\r\nSystem.Object Invoke(System.Object obj, System.Object[] parameters)\r\nvoid _MemberInfo.Invoke(uint32 dispIdMember, [ref] guid riid, uint32 lcid, int16 wFlags, System.IntPtr\r\npDispParams, System.IntPtr pVarResult, System.IntPtr pExcepInfo, System.IntPtr puArgErr)\r\nvoid _MethodBase.Invoke(uint32 dispIdMember, [ref] guid riid, uint32 lcid, int16 wFlags, System.IntPtr\r\npDispParams, System.IntPtr pVarResult, System.IntPtr pExcepInfo, System.IntPtr puArgErr)\r\nSystem.Object _MethodBase.Invoke(System.Object obj, System.Reflection.BindingFlags invokeAttr,\r\nSystem.Reflection.Binder binder, System.Object[] parameters, cultureinfo culture)\r\nSystem.Object _MethodBase.Invoke(System.Object obj, System.Object[] parameters)\r\nvoid _MethodInfo.Invoke(uint32 dispIdMember, [ref] guid riid, uint32 lcid, int16 wFlags, System.IntPtr\r\npDispParams, System.IntPtr pVarResult, System.IntPtr pExcepInfo, System.IntPtr puArgErr)\r\nSystem.Object _MethodInfo.Invoke(System.Object obj, System.Reflection.BindingFlags invokeAttr,\r\nSystem.Reflection.Binder binder, System.Object[] parameters, cultureinfo culture)\r\nSystem.Object _MethodInfo.Invoke(System.Object obj, System.Object[] parameters)\r\n\r\n# this gives me a list of defintions - good! - the second of these definitions seems to be what I am after, so lets try that\r\n# that definition says Invoke() takes two arguments - the object itself (reflection!) and an array of parameters to it\r\nPS> $date.GetType().GetMethods()[24].Invoke($date, 2)\r\n\r\nFriday, March 13, 2015 12:00:00 AM\r\n\r\nPS>\r\n<\/pre>\n
As you can see, without knowing anything about the object we have managed to identify its type, the methods & properties it contains, and even invoke one of the methods. That’s reflection!<\/p>\n
Now I have to figure how to use this info to get a list of accelerators in PowerShell. Which is the topic for another post, of course …<\/p>\n","protected":false},"excerpt":{"rendered":"
I knew that PowerShell constructs such [bool] – which defines the boolean data type – were called accelerators, but I never knew why and didn’t bother much as I figured I’d learn more about them soon or later. But yesterday while reading Bruce Payette’s “Windows PowerShell in Action” that book too mentioned the word accelerator … Continue reading What is Reflection?<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","enabled":false}}},"categories":[2],"tags":[20,104],"jetpack_publicize_connections":[],"jetpack_sharing_enabled":true,"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/rakhesh.com\/wp-json\/wp\/v2\/posts\/696"}],"collection":[{"href":"https:\/\/rakhesh.com\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rakhesh.com\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rakhesh.com\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/rakhesh.com\/wp-json\/wp\/v2\/comments?post=696"}],"version-history":[{"count":0,"href":"https:\/\/rakhesh.com\/wp-json\/wp\/v2\/posts\/696\/revisions"}],"wp:attachment":[{"href":"https:\/\/rakhesh.com\/wp-json\/wp\/v2\/media?parent=696"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rakhesh.com\/wp-json\/wp\/v2\/categories?post=696"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rakhesh.com\/wp-json\/wp\/v2\/tags?post=696"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}