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How to apply an attribute to all classes who inherit monobehaviour in a project/ an assembly?

I'm asking this because I want to eliminate the "Failed to insert item" warnings in a Unity project, which is caused by amount of scripts exceeding a certain value. Here is a solution provided by unity, which suggests applying [AddComponentMenu("")]attribute to all monobehaviours accordingly.
Since I'm working on a project with lots of scripts, it seems very difficult to apply this attribute manually. I need to:
Finding all classes under \Assets\Scripts (or in Assembly-CSharp) who inherit monobehaviour or any of its child class
Applying [AddComponentMenu("PATH")] attibute to the classes found above and put the realtive path of the script in the "PATH"
I don't know how to implement these and have been failing to find a solution by myself.
Some additional features would be wonderful (not necessary):
If there is existing [AddComponentMenu("")] in the script, do not override the original one.
Automatically applying [AddComponentMenu("")] to the scripts to be created.
Looking forward to all kinds of help. Thanks a lot!

There are probably many ways to do this outside of Unity. However, I will assume you want to do it all in Unity itself just for the simpleness of things.
Remark: There are for sure edge-cases left but I hope this is a good starting point
Finding all classes under \Assets\Scripts (or in Assembly-CSharp) who inherit monobehaviour or any of its child class
This first point would be quite simple - kind of. The Assets don't allow to find script types themselves but you can get all script assets by searching for t:MonoScript in the search bar in the Assets view.
The same can be done also via script using AssetDatabase.FindAssets like e.g.
var scriptGUIDs = AssetDataBase.FindAssets($"t:{nameof(MonoScript)}", new[] {"Assets/Scripts"});
From there you will need to load the script asset and check its type using e.g.
foreach (var scriptGUID in scriptGUIDs)
{
// e.g. Assets/Scripts/MyComponent.cs
var scriptAssetPath = AssetDatabase.GUIDToAssetPath(scriptGUID);
// first get the actual asset
var scriptAsset = AssetDatabase.LoadAssetAtPath<MonoScript>(scriptAssetPath);
// get the system type
var scriptType = scriptAsset.GetClass();
// Now first of all check if this is actually something derived from MonoBehaviour -> if not skip
if(!scriptType.IsSubclassOf(typeof(MonoBehaviour))) continue;
.....
}
Applying [AddComponentMenu("PATH")] attribute to the classes found above and put the realtive path of the script in the "PATH"
This is of course way more tricky since it requires to find the according code line (right above the class implementation) and add text into the file.
So this requires multiple steps
Check if this type already has the attribute -> skip, which also solves
If there is existing [AddComponentMenu("")] in the script, do not override the original one.
Load the raw text content
find the line of code which contains public class TYPENAME :
Insert your attribute line before that line
Write all lines back to the file
After all done refresh the data base to cause a reload/recompilation
This could look somewhat like e.g.
// Find all assets of type "MonoScript" in the folder Assets/Scripts
// or remove /Scripts if you really want to go for all in the Assets
var scriptGUIDs = AssetDatabase.FindAssets($"t:{nameof(MonoScript)}", new[] {"Assets/Scripts"});
foreach (var scriptGUID in scriptGUIDs)
{
// e.g. Assets/Scripts/MyComponent.cs
var scriptAssetPath = AssetDatabase.GUIDToAssetPath(scriptGUID);
// first get the actual asset
var scriptAsset = AssetDatabase.LoadAssetAtPath<MonoScript>(scriptAssetPath);
// get the system type
var scriptType = scriptAsset.GetClass();
// Now first of all check if this is actually something derived from MonoBehaviour -> if not skip
if(!scriptType.IsSubclassOf(typeof(MonoBehaviour))) continue;
// check if this class already has the attribute -> if so skip
if(scriptType.IsDefined(typeof(AddComponentMenu), true)) continue;
// otherwise load in the lines as a list so we can insert our new line
var lines = File.ReadAllLines(scriptAssetPath).ToList();
// find the one defining the class
var indexOfClassLine = lines.FindIndex(l => l.Contains($" class {scriptType.Name} : "));
// e.g. Assets/Scripts/MyComponent
// => you might want to e.g. also remove the "Assets" part ;)
var relativePathWithoutExtension = scriptAssetPath.Substring(0, scriptAssetPath.Length - 3);
// insert the new attribute one line right above that line
lines.Insert(indexOfClassLine, $"[AddComponentMenu(\"{relativePathWithoutExtension}\")]");
File.WriteAllLines(scriptAssetPath, lines);
}
AssetDatabase.Refresh();
And finally
Automatically applying [AddComponentMenu("")] to the scripts to be created.
Somewhere in you project you need an implementation of AssetModificationProcessor which will basically do the same thing as before, just this time it is slightly easier since we don't need to check the type since all c# scrips created via the menu are MonoBehaviour by default
public class ScriptKeywordProcessor : UnityEditor.AssetModificationProcessor
{
public static void OnWillCreateAsset(string path)
{
// ignore meta files
var tempPath = path.Replace(".meta", "");
var index = tempPath.LastIndexOf(".");
if (index < 0) return;
// ignore if not a .cs script
var file = tempPath.Substring(index);
if (file != ".cs") return;
// otherwise load in the lines as a list so we can insert our new line
var lines = File.ReadAllLines(path).ToList();
// find the one defining the class
var indexOfClassLine = lines.FindIndex(l => l.Contains($" class {Path.GetFileNameWithoutExtension(path)} : "));
// e.g. Assets/Scripts/MyComponent
// => you might want to e.g. also remove the "Assets" part ;)
var relativePathWithoutExtension = path.Substring(0, path.Length - 3);
// insert the new attribute one line right above that line
lines.Insert(indexOfClassLine, $"[AddComponentMenu(\"{relativePathWithoutExtension}\")]");
File.WriteAllLines(path, lines);
AssetDatabase.Refresh();
}
}

C# deserialize a class - the "default" value of a variable

I save local user settings to xml file. Program contains "Settings" class that serialize when the program is closed and deserialize when it is started next time.
But the problem is that the program is changed all the time, and when I create next version - I want the user settings to be saved. But the program may contains new fields of settings, and then the program will started and deserialised the old xml file - new fields will be null.
Now I check every fields as hard-code in the program, as like:
Settings sts = (Settings)Deserialise(path);
if(sts.Field2 == null) sts.Field2 = "defaultvalue2";
if(sts.Field3 == null) sts.Field3 = "defaultvalue3";
Of course it is not satisfied for me. Is it possible to do "default" value of a variable as the same time when I change code of Settings class? Like this:
class Settings
{
public string Field1 (DefaultValue: "defaultvalue1");
public string Field2 (DefaultValue: "defaultvalue2");
}
public void Main
{
Settings sts = (Settings)Deserialise(path);
foreach(var fld in typeof(sts))
{
if(fld.Value == null)
fld.Value = Settings.Fields[fld].DefaulValue;
}
}

Yes it is possible, simply use the standard way to set standard values:
class Settings
{
public string Field1 = "defaultvalue1";
public string Field2 = "defaultvalue2";
}
public void Main
{
Settings sts = (Settings)Deserialise(path);
/* not needed
foreach(var fld in typeof(sts))
{
if(fld.Value == null)
fld.Value = Settings.Fields[fld].DefaulValue;
}*/
}

https://learn.microsoft.com/zh-tw/dotnet/api/system.xml.serialization.xmlattributes.xmldefaultvalue?view=netcore-3.1
here I google it . maybe try it?
Use default attribute : DefaultValueAttribute
public class Pet
{
// The default value for the Animal field is "Dog".
[DefaultValueAttribute("Dog")]
public string Animal ;
}

The Settings.settings xml file was designed for static project settings and, using user scoped settings, can be saved at runtime. Are you changing the settings so much that it no longer have the 'old' values or just adding to the list of settings?
If just adding, you don't need to loop through the settings one by one and try to guess their types with values as you can just do this:
int myInteger = Properties.Settings.Default.MyIntegerSettingValue;
And writing to the settings file:
Properties.Settings.Default.MyIntegerSettingValue = myInteger;
So if you cannot replace your settings.xml file, my suggestion is to model your settings to a class that contain all of your settings loaded at runtime and for each one missing, just write it out to the Settings file with your default value:
Properties.Settings.Default.MyMissingSetting = "MyDefaultValue"
You can find some nice info on application settings usage here

How to get the file author using C# [duplicate]

I'm trying to find out how to read/write to the extended file properties in C#
e.g. Comment, Bit Rate, Date Accessed, Category etc that you can see in Windows explorer.
Any ideas how to do this?
EDIT: I'll mainly be reading/writing to video files (AVI/DIVX/...)

For those of not crazy about VB, here it is in c#:
Note, you have to add a reference to Microsoft Shell Controls and Automation from the COM tab of the References dialog.
public static void Main(string[] args)
{
List<string> arrHeaders = new List<string>();
Shell32.Shell shell = new Shell32.Shell();
Shell32.Folder objFolder;
objFolder = shell.NameSpace(#"C:\temp\testprop");
for( int i = 0; i < short.MaxValue; i++ )
{
string header = objFolder.GetDetailsOf(null, i);
if (String.IsNullOrEmpty(header))
break;
arrHeaders.Add(header);
}
foreach(Shell32.FolderItem2 item in objFolder.Items())
{
for (int i = 0; i < arrHeaders.Count; i++)
{
Console.WriteLine(
$"{i}\t{arrHeaders[i]}: {objFolder.GetDetailsOf(item, i)}");
}
}
}

Solution 2016
Add following NuGet packages to your project:
Microsoft.WindowsAPICodePack-Shell by Microsoft
Microsoft.WindowsAPICodePack-Core by Microsoft
Read and Write Properties
using Microsoft.WindowsAPICodePack.Shell;
using Microsoft.WindowsAPICodePack.Shell.PropertySystem;
string filePath = #"C:\temp\example.docx";
var file = ShellFile.FromFilePath(filePath);
// Read and Write:
string[] oldAuthors = file.Properties.System.Author.Value;
string oldTitle = file.Properties.System.Title.Value;
file.Properties.System.Author.Value = new string[] { "Author #1", "Author #2" };
file.Properties.System.Title.Value = "Example Title";
// Alternate way to Write:
ShellPropertyWriter propertyWriter = file.Properties.GetPropertyWriter();
propertyWriter.WriteProperty(SystemProperties.System.Author, new string[] { "Author" });
propertyWriter.Close();
Important:
The file must be a valid one, created by the specific assigned software. Every file type has specific extended file properties and not all of them are writable.
If you right-click a file on desktop and cannot edit a property, you wont be able to edit it in code too.
Example:
Create txt file on desktop, rename its extension to docx. You can't
edit its Author or Title property.
Open it with Word, edit and save
it. Now you can.
So just make sure to use some try catch
Further Topic:
Microsoft Docs: Implementing Property Handlers

There's a CodeProject article for an ID3 reader. And a thread at kixtart.org that has more information for other properties. Basically, you need to call the GetDetailsOf() method on the folder shell object for shell32.dll.

This sample in VB.NET reads all extended properties:
Sub Main()
Dim arrHeaders(35)
Dim shell As New Shell32.Shell
Dim objFolder As Shell32.Folder
objFolder = shell.NameSpace("C:\tmp")
For i = 0 To 34
arrHeaders(i) = objFolder.GetDetailsOf(objFolder.Items, i)
Next
For Each strFileName In objfolder.Items
For i = 0 To 34
Console.WriteLine(i & vbTab & arrHeaders(i) & ": " & objfolder.GetDetailsOf(strFileName, i))
Next
Next
End Sub
You have to add a reference to Microsoft Shell Controls and Automation from the COM tab of the References dialog.

Thank you guys for this thread! It helped me when I wanted to figure out an exe's file version. However, I needed to figure out the last bit myself of what is called Extended Properties.
If you open properties of an exe (or dll) file in Windows Explorer, you get a Version tab, and a view of Extended Properties of that file. I wanted to access one of those values.
The solution to this is the property indexer FolderItem.ExtendedProperty and if you drop all spaces in the property's name, you'll get the value. E.g. File Version goes FileVersion, and there you have it.
Hope this helps anyone else, just thought I'd add this info to this thread. Cheers!

GetDetailsOf() Method - Retrieves details about an item in a folder. For example, its size, type, or the time of its last modification. File Properties may vary based on the Windows-OS version.
List<string> arrHeaders = new List<string>();
Shell shell = new ShellClass();
Folder rFolder = shell.NameSpace(_rootPath);
FolderItem rFiles = rFolder.ParseName(filename);
for (int i = 0; i < short.MaxValue; i++)
{
string value = rFolder.GetDetailsOf(rFiles, i).Trim();
arrHeaders.Add(value);
}

Jerker's answer is little simpler. Here's sample code which works from MS:
var folder = new Shell().NameSpace(folderPath);
foreach (FolderItem2 item in folder.Items())
{
var company = item.ExtendedProperty("Company");
var author = item.ExtendedProperty("Author");
// Etc.
}
For those who can't reference shell32 statically, you can invoke it dynamically like this:
var shellAppType = Type.GetTypeFromProgID("Shell.Application");
dynamic shellApp = Activator.CreateInstance(shellAppType);
var folder = shellApp.NameSpace(folderPath);
foreach (var item in folder.Items())
{
var company = item.ExtendedProperty("Company");
var author = item.ExtendedProperty("Author");
// Etc.
}

After looking at a number of solutions on this thread and elsewhere
the following code was put together. This is only to read a property.
I could not get the
Shell32.FolderItem2.ExtendedProperty function to work, it is supposed
to take a string value and return the correct value and type for that
property... this was always null for me and developer reference resources were very thin.
The WindowsApiCodePack seems
to have been abandoned by Microsoft which brings us the code below.
Use:
string propertyValue = GetExtendedFileProperty("c:\\temp\\FileNameYouWant.ext","PropertyYouWant");
Will return you the value of the extended property you want as a
string for the given file and property name.
Only loops until it found the specified property - not until
all properties are discovered like some sample code
Will work on Windows versions like Windows server 2008 where you will get the error "Unable to cast COM object of type 'System.__ComObject' to interface type 'Shell32.Shell'" if just trying to create the Shell32 Object normally.
public static string GetExtendedFileProperty(string filePath, string propertyName)
{
string value = string.Empty;
string baseFolder = Path.GetDirectoryName(filePath);
string fileName = Path.GetFileName(filePath);
//Method to load and execute the Shell object for Windows server 8 environment otherwise you get "Unable to cast COM object of type 'System.__ComObject' to interface type 'Shell32.Shell'"
Type shellAppType = Type.GetTypeFromProgID("Shell.Application");
Object shell = Activator.CreateInstance(shellAppType);
Shell32.Folder shellFolder = (Shell32.Folder)shellAppType.InvokeMember("NameSpace", System.Reflection.BindingFlags.InvokeMethod, null, shell, new object[] { baseFolder });
//Parsename will find the specific file I'm looking for in the Shell32.Folder object
Shell32.FolderItem folderitem = shellFolder.ParseName(fileName);
if (folderitem != null)
{
for (int i = 0; i < short.MaxValue; i++)
{
//Get the property name for property index i
string property = shellFolder.GetDetailsOf(null, i);
//Will be empty when all possible properties has been looped through, break out of loop
if (String.IsNullOrEmpty(property)) break;
//Skip to next property if this is not the specified property
if (property != propertyName) continue;
//Read value of property
value = shellFolder.GetDetailsOf(folderitem, i);
}
}
//returns string.Empty if no value was found for the specified property
return value;
}

Here is a solution for reading - not writing - the extended properties based on what I found on this page and at help with shell32 objects.
To be clear this is a hack. It looks like this code will still run on Windows 10 but will hit on some empty properties. Previous version of Windows should use:
var i = 0;
while (true)
{
...
if (String.IsNullOrEmpty(header)) break;
...
i++;
On Windows 10 we assume that there are about 320 properties to read and simply skip the empty entries:
private Dictionary<string, string> GetExtendedProperties(string filePath)
{
var directory = Path.GetDirectoryName(filePath);
var shell = new Shell32.Shell();
var shellFolder = shell.NameSpace(directory);
var fileName = Path.GetFileName(filePath);
var folderitem = shellFolder.ParseName(fileName);
var dictionary = new Dictionary<string, string>();
var i = -1;
while (++i < 320)
{
var header = shellFolder.GetDetailsOf(null, i);
if (String.IsNullOrEmpty(header)) continue;
var value = shellFolder.GetDetailsOf(folderitem, i);
if (!dictionary.ContainsKey(header)) dictionary.Add(header, value);
Console.WriteLine(header +": " + value);
}
Marshal.ReleaseComObject(shell);
Marshal.ReleaseComObject(shellFolder);
return dictionary;
}
As mentioned you need to reference the Com assembly Interop.Shell32.
If you get an STA related exception, you will find the solution here:
Exception when using Shell32 to get File extended properties
I have no idea what those properties names would be like on a foreign system and couldn't find information about which localizable constants to use in order to access the dictionary. I also found that not all the properties from the Properties dialog were present in the dictionary returned.
BTW this is terribly slow and - at least on Windows 10 - parsing dates in the string retrieved would be a challenge so using this seems to be a bad idea to start with.
On Windows 10 you should definitely use the Windows.Storage library which contains the SystemPhotoProperties, SystemMusicProperties etc.
https://learn.microsoft.com/en-us/windows/uwp/files/quickstart-getting-file-properties
And finally, I posted a much better solution that uses WindowsAPICodePack there

I'm not sure what types of files you are trying to write the properties for but taglib-sharp is an excellent open source tagging library that wraps up all this functionality nicely. It has a lot of built in support for most of the popular media file types but also allows you to do more advanced tagging with pretty much any file.
EDIT: I've updated the link to taglib sharp. The old link no longer worked.
EDIT: Updated the link once again per kzu's comment.

Navigating to DNN Module

I'm forming a newsletter with links to various html modules within my DNN website. I have access to each of their ModuleID's and I'm wanting to use that to get the url. The current approach (made by a third party developer) worked, but only to a degree. The url's are incorrectly formed when the Modules are located deeper in the website.
For example module located at www.website.com/website/articles.aspx is works fine, but a module located www.website.com/website/articles/subarticles.aspx won't. I know this is because the url is incorrectly formed.
Here's the current code:
DotNetNuke.Entities.Modules.ModuleController objModCtrlg = new DotNetNuke.Entities.Modules.ModuleController();
DotNetNuke.Entities.Modules.ModuleInfo dgfdgdg = objModCtrlg.GetModule(ContentMID);
TabController objtabctrll = new TabController();
TabInfo objtabinfoo = objtabctrll.GetTab(tabidfrcontent);
string tabnamefremail= objtabinfoo.TabName;
moduletitlefrEmail = dgfdgdg.ModuleTitle;
string readmorelinkpath = basePath + "/" + tabnamefremail + ".aspx";
ContentMID is the current module ID I'm looking at. I've tried to use Globals.NavigateURL, but that always crashes with Object reference not set to an instance of an object. error. Same thing when I use objtabinfoo.FullUrl so I'm currently at a loss as to how I get the specific modules URL.
EDIT: Here's some more code as to how the tabId is retrieved.
IDictionary<int, TabInfo> dicTabInfo12 = new Dictionary<int, TabInfo>();
ContentMID = Convert.ToInt32(dsNewsList.Tables[0].Rows[i]["ModuleID"]);
dicTabInfo12 = objTabctrl.GetTabsByModuleID(ContentMID);
if (dicTabInfo12.Count > 0)
{
string tester = ""; //Debug
foreach (KeyValuePair<int, TabInfo> item1 in dicTabInfo12)
{
tabidfrcontent = item1.Key;
}
}

You really should be using NavigateUrl to build the links ance if you have the tabid, you are golden.
string readMoreLinkPath = NavigateUrl(tabidfrcontent);
Nice and simple

Okay, colleague suggested this and it works great within a scheduler.
string linkPath = basePath + "/Default.aspx?TabID=" + tabID;
Will Navigate you to the correct tab ID. So this would be the best solution if you're forced to work within a scheduler where you can't use NavigateUrl without some major workarounds.

How do I read and write details from a file in C# [duplicate]

I'm trying to find out how to read/write to the extended file properties in C#
e.g. Comment, Bit Rate, Date Accessed, Category etc that you can see in Windows explorer.
Any ideas how to do this?
EDIT: I'll mainly be reading/writing to video files (AVI/DIVX/...)

For those of not crazy about VB, here it is in c#:
Note, you have to add a reference to Microsoft Shell Controls and Automation from the COM tab of the References dialog.
public static void Main(string[] args)
{
List<string> arrHeaders = new List<string>();
Shell32.Shell shell = new Shell32.Shell();
Shell32.Folder objFolder;
objFolder = shell.NameSpace(#"C:\temp\testprop");
for( int i = 0; i < short.MaxValue; i++ )
{
string header = objFolder.GetDetailsOf(null, i);
if (String.IsNullOrEmpty(header))
break;
arrHeaders.Add(header);
}
foreach(Shell32.FolderItem2 item in objFolder.Items())
{
for (int i = 0; i < arrHeaders.Count; i++)
{
Console.WriteLine(
$"{i}\t{arrHeaders[i]}: {objFolder.GetDetailsOf(item, i)}");
}
}
}

Solution 2016
Add following NuGet packages to your project:
Microsoft.WindowsAPICodePack-Shell by Microsoft
Microsoft.WindowsAPICodePack-Core by Microsoft
Read and Write Properties
using Microsoft.WindowsAPICodePack.Shell;
using Microsoft.WindowsAPICodePack.Shell.PropertySystem;
string filePath = #"C:\temp\example.docx";
var file = ShellFile.FromFilePath(filePath);
// Read and Write:
string[] oldAuthors = file.Properties.System.Author.Value;
string oldTitle = file.Properties.System.Title.Value;
file.Properties.System.Author.Value = new string[] { "Author #1", "Author #2" };
file.Properties.System.Title.Value = "Example Title";
// Alternate way to Write:
ShellPropertyWriter propertyWriter = file.Properties.GetPropertyWriter();
propertyWriter.WriteProperty(SystemProperties.System.Author, new string[] { "Author" });
propertyWriter.Close();
Important:
The file must be a valid one, created by the specific assigned software. Every file type has specific extended file properties and not all of them are writable.
If you right-click a file on desktop and cannot edit a property, you wont be able to edit it in code too.
Example:
Create txt file on desktop, rename its extension to docx. You can't
edit its Author or Title property.
Open it with Word, edit and save
it. Now you can.
So just make sure to use some try catch
Further Topic:
Microsoft Docs: Implementing Property Handlers

There's a CodeProject article for an ID3 reader. And a thread at kixtart.org that has more information for other properties. Basically, you need to call the GetDetailsOf() method on the folder shell object for shell32.dll.

This sample in VB.NET reads all extended properties:
Sub Main()
Dim arrHeaders(35)
Dim shell As New Shell32.Shell
Dim objFolder As Shell32.Folder
objFolder = shell.NameSpace("C:\tmp")
For i = 0 To 34
arrHeaders(i) = objFolder.GetDetailsOf(objFolder.Items, i)
Next
For Each strFileName In objfolder.Items
For i = 0 To 34
Console.WriteLine(i & vbTab & arrHeaders(i) & ": " & objfolder.GetDetailsOf(strFileName, i))
Next
Next
End Sub
You have to add a reference to Microsoft Shell Controls and Automation from the COM tab of the References dialog.

Thank you guys for this thread! It helped me when I wanted to figure out an exe's file version. However, I needed to figure out the last bit myself of what is called Extended Properties.
If you open properties of an exe (or dll) file in Windows Explorer, you get a Version tab, and a view of Extended Properties of that file. I wanted to access one of those values.
The solution to this is the property indexer FolderItem.ExtendedProperty and if you drop all spaces in the property's name, you'll get the value. E.g. File Version goes FileVersion, and there you have it.
Hope this helps anyone else, just thought I'd add this info to this thread. Cheers!

GetDetailsOf() Method - Retrieves details about an item in a folder. For example, its size, type, or the time of its last modification. File Properties may vary based on the Windows-OS version.
List<string> arrHeaders = new List<string>();
Shell shell = new ShellClass();
Folder rFolder = shell.NameSpace(_rootPath);
FolderItem rFiles = rFolder.ParseName(filename);
for (int i = 0; i < short.MaxValue; i++)
{
string value = rFolder.GetDetailsOf(rFiles, i).Trim();
arrHeaders.Add(value);
}

Jerker's answer is little simpler. Here's sample code which works from MS:
var folder = new Shell().NameSpace(folderPath);
foreach (FolderItem2 item in folder.Items())
{
var company = item.ExtendedProperty("Company");
var author = item.ExtendedProperty("Author");
// Etc.
}
For those who can't reference shell32 statically, you can invoke it dynamically like this:
var shellAppType = Type.GetTypeFromProgID("Shell.Application");
dynamic shellApp = Activator.CreateInstance(shellAppType);
var folder = shellApp.NameSpace(folderPath);
foreach (var item in folder.Items())
{
var company = item.ExtendedProperty("Company");
var author = item.ExtendedProperty("Author");
// Etc.
}

After looking at a number of solutions on this thread and elsewhere
the following code was put together. This is only to read a property.
I could not get the
Shell32.FolderItem2.ExtendedProperty function to work, it is supposed
to take a string value and return the correct value and type for that
property... this was always null for me and developer reference resources were very thin.
The WindowsApiCodePack seems
to have been abandoned by Microsoft which brings us the code below.
Use:
string propertyValue = GetExtendedFileProperty("c:\\temp\\FileNameYouWant.ext","PropertyYouWant");
Will return you the value of the extended property you want as a
string for the given file and property name.
Only loops until it found the specified property - not until
all properties are discovered like some sample code
Will work on Windows versions like Windows server 2008 where you will get the error "Unable to cast COM object of type 'System.__ComObject' to interface type 'Shell32.Shell'" if just trying to create the Shell32 Object normally.
public static string GetExtendedFileProperty(string filePath, string propertyName)
{
string value = string.Empty;
string baseFolder = Path.GetDirectoryName(filePath);
string fileName = Path.GetFileName(filePath);
//Method to load and execute the Shell object for Windows server 8 environment otherwise you get "Unable to cast COM object of type 'System.__ComObject' to interface type 'Shell32.Shell'"
Type shellAppType = Type.GetTypeFromProgID("Shell.Application");
Object shell = Activator.CreateInstance(shellAppType);
Shell32.Folder shellFolder = (Shell32.Folder)shellAppType.InvokeMember("NameSpace", System.Reflection.BindingFlags.InvokeMethod, null, shell, new object[] { baseFolder });
//Parsename will find the specific file I'm looking for in the Shell32.Folder object
Shell32.FolderItem folderitem = shellFolder.ParseName(fileName);
if (folderitem != null)
{
for (int i = 0; i < short.MaxValue; i++)
{
//Get the property name for property index i
string property = shellFolder.GetDetailsOf(null, i);
//Will be empty when all possible properties has been looped through, break out of loop
if (String.IsNullOrEmpty(property)) break;
//Skip to next property if this is not the specified property
if (property != propertyName) continue;
//Read value of property
value = shellFolder.GetDetailsOf(folderitem, i);
}
}
//returns string.Empty if no value was found for the specified property
return value;
}

Here is a solution for reading - not writing - the extended properties based on what I found on this page and at help with shell32 objects.
To be clear this is a hack. It looks like this code will still run on Windows 10 but will hit on some empty properties. Previous version of Windows should use:
var i = 0;
while (true)
{
...
if (String.IsNullOrEmpty(header)) break;
...
i++;
On Windows 10 we assume that there are about 320 properties to read and simply skip the empty entries:
private Dictionary<string, string> GetExtendedProperties(string filePath)
{
var directory = Path.GetDirectoryName(filePath);
var shell = new Shell32.Shell();
var shellFolder = shell.NameSpace(directory);
var fileName = Path.GetFileName(filePath);
var folderitem = shellFolder.ParseName(fileName);
var dictionary = new Dictionary<string, string>();
var i = -1;
while (++i < 320)
{
var header = shellFolder.GetDetailsOf(null, i);
if (String.IsNullOrEmpty(header)) continue;
var value = shellFolder.GetDetailsOf(folderitem, i);
if (!dictionary.ContainsKey(header)) dictionary.Add(header, value);
Console.WriteLine(header +": " + value);
}
Marshal.ReleaseComObject(shell);
Marshal.ReleaseComObject(shellFolder);
return dictionary;
}
As mentioned you need to reference the Com assembly Interop.Shell32.
If you get an STA related exception, you will find the solution here:
Exception when using Shell32 to get File extended properties
I have no idea what those properties names would be like on a foreign system and couldn't find information about which localizable constants to use in order to access the dictionary. I also found that not all the properties from the Properties dialog were present in the dictionary returned.
BTW this is terribly slow and - at least on Windows 10 - parsing dates in the string retrieved would be a challenge so using this seems to be a bad idea to start with.
On Windows 10 you should definitely use the Windows.Storage library which contains the SystemPhotoProperties, SystemMusicProperties etc.
https://learn.microsoft.com/en-us/windows/uwp/files/quickstart-getting-file-properties
And finally, I posted a much better solution that uses WindowsAPICodePack there

I'm not sure what types of files you are trying to write the properties for but taglib-sharp is an excellent open source tagging library that wraps up all this functionality nicely. It has a lot of built in support for most of the popular media file types but also allows you to do more advanced tagging with pretty much any file.
EDIT: I've updated the link to taglib sharp. The old link no longer worked.
EDIT: Updated the link once again per kzu's comment.