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in an IIS web app, how do I get the windows user name? - not the process username

I have inherited an old asp.net web app. Part of the login process is intended to collect the windows username (not the process username). This used to work (years ago) but a new compile of the system returns the App Pool name / username instead of the windows username.
The username is collected before authentication by the website, originally using HttpContext.Current.User.Identity.Name
I have tried all the other options that have been suggested with no change in the results.
string loginName;
loginName = Environment.UserName;
loginName = HttpContext.Current.User.Identity.Name;
loginName = System.Windows.Forms.SystemInformation.UserName;
loginName = System.Security.Principal.WindowsIdentity.GetCurrent().Name;
loginName = System.DirectoryServices.AccountManagement.UserPrincipal.Current.SamAccountName;
loginName = System.DirectoryServices.AccountManagement.UserPrincipal.Current.UserPrincipalName;
I tried these one at a time (not all together) and in the target environment only ever got the App Pool name or App Pool username.
I have tried every combination of Authentication in IIS, the only one that makes a difference is ASP.NET impersonation which changes from App Pool name to impersonation user name.
On my local PC, launched out of Visual Studio, they all work to return the windows logged in username.
I'm quite new to asp.net web apps, and I don't know what else to try.

You have stumbled on the answer. If you want the server to automatically know the windows user, that windows user's credentials need to be passed to the application. If you use the Kerberos or NTML authentication options (Windows Authentication) that set of credentials is passed automatically. Otherwise you need to prompt the user to enter credentials. You don't have to use impersonation, but you can programmatically impersonate the logged in user if your app pool user has sufficient access to the dc catalog. More on that here: https://learn.microsoft.com/en-us/previous-versions/msp-n-p/ff647405(v=pandp.10)#impersonating-the-original-caller-programmatically

How to get currently Windows logged User Credentials?

I am working on an ASP.NET Core 2.0 web app. It has it's login and own User interface.
I am trying to retrieve Windows logged users' credentials - like user name, email and password. On login in app, the logged in user's credentials are set as the User. So I need to retrieve windows logged in user's info before login process. I tried the following :
// If domain is example.com, DomainName gives EXAMPLECOM. Plus can't access Password
string evnName = Environment.UserDomainName + Environment.UserName;
In HomeController (after login) and Startup, I tried
// User is null
string userName = this.User.FindFirst(ClaimTypes.Name)?.Value;
I tried the above in a Service where login process is processed. Can't get from their too.
The main reason to retrieve this is to identify if the user has privileges for SMTP access or not, if so set SMTP.
Can anyone please help me know, how can I retrieve Windows Logged User's credentials somehow before Login process.
Thanks a lot.

You can get the current Windows user credentials by using the System.Net.CredentialCache.DefaultCredentials static property.
You won't be able to get the password as that would be a major security hole. You should be able to get everything else you want out of that, however.

Getting Client side Windows User ID in asp.net, for LDAP single sign on

We have an existing webform that allows that uses form authentication that requires the user to key in id and pw to login to the system. Client requested that the program just take the username from the windows logged on username, and by this I mean the username used to login to the computer, not to the webpage.
Methods I have used to try to get the username:
System.Environment.UserName;
HttpContext.Current.User.Identity.Name;
Context.User.Identity.Name;
System.Security.Principal.WindowsIdentity.GetCurrent().Name;
Page.User.Identity.Name;
Windows Authentication(WA), Forms Authentication(FA), Basic Authentication(BA), Anonymous Authentication(AA), ASP.NET Impersonation (AI) have been enabled disabled in multiple different pairs. The suggestion I found the most were to enable (WA), disable (AA), enable (AI), but to no avail. The only thing that seems to affect the codes above is AI from my testing.
Enabling (AI) gives me:
NT AUTHORITY\IUSR
Disabling (AI) gives me:
IIS APPPOOL\ASP.NET V4.0 Integrated
Whereas what I am trying to get is MyDomain\MyUserName
I tried accessing the page from another computer that is not hosting it but still in the same domain and it gave me the same output. That lead me to suspect that the codes were giving me only the user that is running the process in the server, instead of the client side's Windows user.
My question is, does anyone have any idea how to get the username of the person logged in to the domain?
*Additional info: I'm using super user account in the domain, and providing the admin account and password to run visual studio as admin, if that somehow affects anything.

You have to enable Windows authentication in IIS, and disable other authentication methods.
And make sure to set the authentication mode as "Windows" in web.config.
<authentication mode="Windows"/>
Hope it helps!

I have a windows authenticated HTML page hosted on IIS. How to get LoginID using C# code

I have created a sample html page and hosted it on IIS. I have enabled windows authentication. Now what I want to do is whenever I run the application, the Login ID entered by the user to be stored in database (I have already created a web service to send data to my database). I just want know the command to store the Login ID in a variable. Kindly assist.

Try this
WindowsIdentity yourUserIdentity = HttpContext.Current.Request.LogonUserIdentity;
Now you can get the User name by the following property of the WindowsIdentity object yourUserIdentity.
yourUserIdentity.Name

This should get you the username:
System.Web.HttpContext.Current.User.Identity.Name
Here is an explanation on how it works:
http://weblogs.asp.net/scottgu/Recipe_3A00_-Enabling-Windows-Authentication-within-an-Intranet-ASP.NET-Web-application

if you are using windows authentication you can use this to get the LoginID
string user = Request.ServerVariables["LOGON_USER"];

Get system logged in username in c# that also works in IIS

I thought its simple to get the logged in username, in fact, it works fine with most of the responses available on stack overflow but when I publish the website all method fails.
Can anybody please guide me to get the name of the logged in user from Windows with following condition.
I just need logged in user name so forget about AD when you write the response.
I cannot change to Windows authentication mode because user may be a guest user who is not part of company but still have access to my intranet website
it’s not possible to change the user browser setting as there are more than 6000 users
I cannot disable the anonymous authentication as I want everyone to be able to use the website
I have already tried following solution and all works fine when I run the website on debug mode but all fails to return the username when I publish the website on IIS so please help me with some new as solution
string _windowLogonUserName = System.Environment.UserName.ToString()
string _windowLogonUserName =
WindowsIdentity.GetCurrent().Name.Remove(0, _adDomainName.Length + 1)
string _windowLogonUserName =
System.Web.HttpContext.Current.User.Identity.Name.ToString();
System.Security.Principal.WindowsIdentity.GetCurrent().Name;
AppDomain appDomain = Thread.GetDomain();
appDomain.SetPrincipalPolicy(PrincipalPolicy.WindowsPrincipal);
WindowsPrincipal windowsPrincipal = (WindowsPrincipal)Thread.CurrentPrincipal;
Logger.Write("username principal" + windowsPrincipal.Identity.Name);
Request.ServerVariables["LOGON_USER"]

It sounds like you're publishing the site outside of the AD domain. if the server is not on the domain, it won't be able to authenticate (or even accept) Windows users. The basic answer is that you can't get it.
If the server is internal (on the domain) then you can enabled integrated windows auth in IIS. Here's a reference.