Authenticating user/password against Active Directory using ASP.NET Identity

SignInAsync is an async method, and you most likely want to wait for it to complete before returning the SignInStatus. In order to await it, we must declare the method with the async keyword. This also means we can simplify the return statements:

public override async Task<SignInStatus> PasswordSignInAsync(string userName, string password, bool isPersistent, bool shouldLockout)
{
    if (Membership.Provider.ValidateUser(userName, password))
    {
        var user = UserManager.FindByName(userName);
        if (user == null)
        {
            return SignInStatus.Failure;
        }

        await base.SignInAsync(user, isPersistent, shouldLockout);
        return SignInStatus.Success;
    }

    return SignInStatus.Failure;
}

Try using ContinueWith.

I cannot test the Membership Provider part at this time, but the ContinueWith I use in a similar fashion in this exact same method. In your situation PasswordSignInAsync will fail and will give control to ContinueWith and there you can ignore the passed in signInStatus and put your own custom code. Your situation is different in that you don't need to run the base.PasswordSignInAsync(). If so, maybe it is useful to others needing it like I did. In the ContinueWith block I had to put some custom code to run things against our own security framework.

    public override Task<SignInStatus> PasswordSignInAsync(string userName, string password, bool isPersistent, bool shouldLockout)
    {
        return base.PasswordSignInAsync(userName, password, isPersistent, shouldLockout).ContinueWith(signInStatus =>
        {
            if (signInStatus.Result == SignInStatus.Failure)
            {
                if (Membership.Provider.ValidateUser(userName, password))
                {
                    var user = UserManager.FindByName(userName);
                    if (user == null)
                    {
                        return SignInStatus.Failure;
                    }

                    base.SignInAsync(user, isPersistent, shouldLockout);
                    return SignInStatus.Success;
                }
            }

            return signInStatus.Result;
        });
    }