Log in using AD credentials in C#

I am using following code to validate AD users

string strLDAP = "LDAP://dc=ADServerIP/cn=Users,DC=Domain;
DirectoryEntry entry = new DirectoryEntry(strLDAP, usr, pwd);
object nativeObject = entry.NativeObject;
return true;

I am getting the following exception when executing

object nativeObject = entry.NativeObject;

System.Runtime.InteropServices.COMException (0x80005000): Unknown error (0x80005000)
at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail)
at System.DirectoryServices.DirectoryEntry.Bind()
at System.DirectoryServices.DirectoryEntry.get_NativeObject()

The same code is working for another AD server. What may be the issue?

Are you working on .NET 3.5 or newer? If so, you can use the System.DirectoryServices.AccountManagement namespace and easily verify your credentials:

// create a "principal context" - e.g. your domain (could be machine, too)
using(PrincipalContext pc = new PrincipalContext(ContextType.Domain, "YOURDOMAIN", usr, pwd))
{
    // validate the credentials
    bool isValid = pc.ValidateCredentials("myuser", "mypassword");
}

It's simple, it's reliable, it's 100% C# managed code on your end - what more can you ask for? :-)

Read all about it here:

• Managing Directory Security Principals in the .NET Framework 3.5
• MSDN docs on System.DirectoryServices.AccountManagement