How to validate windows local users using c# ?

Hi,

I am using below code to validate but it is working for few users and it is not working for few users.please help me to resolve this issue.

public bool ValidatePassword(string userName, string Password, ref string errorMessage)
{
errorMessage = "";
PrincipalContext principalContext = null;
UserPrincipal userPrincipal = null;

            try
            {


                principalContext = new PrincipalContext(ContextType.Machine, null, null, userName, Password);

                userPrincipal = UserPrincipal.FindByIdentity(principalContext, userName);

                if (userPrincipal != null)
                {
                    userPrincipal.Dispose();
                    principalContext.Dispose();
                    return true;
                }
                else
                {
                    errorMessage = "Invalid user / password";
                    return false;
                }
            }
            catch (Exception ex)
            {

                if (userPrincipal != null)
                    userPrincipal.Dispose();

                if (principalContext != null)
                    principalContext.Dispose();

                errorMessage = ex.Message;
                System.Diagnostics.Debug.WriteLine("Error validate password: {0}", ex.Message);

                return false;
            }
        }

Thanks & regards,
Venkateswara rao