title: Validate a username and password against AD exception
body: <p>I have the following code to verify a Active Directory user(VS2008 on Win7).</p>
  
  <pre><code>try
  {
      PrincipalContext pc = new PrincipalContext(ContextType.Domain, strDomainName))
      isPass = pc.ValidateCredentials(strDomainUserName, strDomainUserPass);
  }
  catch (Exception ex)
  {
      Logger.Log("authentiate domain user fail: {0},{1}", ex.ToString(), strDomainUserLoginName);
  }
  </code></pre>
  
  <p>For some AD user, it is OK, but for a AD user, I will get the following exception:
  <code>System.DirectoryServices.Protocols.DirectoryOperationException:</code> <code>The server cannot handle directory requests.</code></p>
  
  <pre><code>System.DirectoryServices.Protocols.ErrorChecking.CheckAndSetLdapError(Int32 error)
  
  System.DirectoryServices.Protocols.LdapSessionOptions.FastConcurrentBind()
  
  System.DirectoryServices.AccountManagement.CredentialValidator.BindLdap(NetworkCredential creds, ContextOptions contextOptions)
  
  System.DirectoryServices.AccountManagement.CredentialValidator.Validate(String userName, String password)
  
  System.DirectoryServices.AccountManagement.PrincipalContext.ValidateCredentials(String userName, String password)
  </code></pre>
  
  <p>So does Active Directory Domain user setting will cause such a exception?</p>