public bool EmailValidation(string UserName, string password)
        {
            var x = (from n in db.LoginCrediential
                    where n.UserName == UserName && n.Password == password
                    select n);
            if (x != null)
                return true;
            else
                return false;
        }

public class MyAuthorizationServerProvider: OAuthAuthorizationServerProvider
{
    ICrendentials objcredential = new Crendentials();
    LoginCrediential objUser = new LoginCrediential();

    public override async Task ValidateClientAuthentication(OAuthValidateClientAuthenticationContext context)
    {
        context.Validated(); //
    }
    public override async Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
    {
        var identity = new ClaimsIdentity(context.Options.AuthenticationType);

         if (context.UserName == objUser.UserName && context.Password == objUser.Password)
        {
            var x = objcredential.EmailValidation(context.UserName, context.Password);
            if (x==true)
            {

            }
        }
        else
        {
            context.SetError("invalid_grant", "Provided username and password is incorrect");
            return;
        }

    }