Way to write Validation tests for Login screen in CODEDUI

Being a bignner in CodedUI i have small query suppose we write some UI tests for "Login" Screen in UIMap class like below

public void IsValidUser()
        {
            //Define sample app Main window
            WpfWindow mainWindow = new WpfWindow();
            mainWindow.SearchProperties.Add(WpfWindow.PropertyNames.Name, "ValidatingControlsView");
            mainWindow.SearchProperties.Add(WpfWindow.PropertyNames.ControlType, "Window");
            mainWindow.WindowTitles.Add("ValidatingControlsView");

            //Username text box
            WpfEdit txtusrname = new WpfEdit(mainWindow);
            txtusrname.SearchProperties.Add(WpfButton.PropertyNames.AutomationId, "txtUsername");
            txtusrname.WindowTitles.Add("ValidatingControlsView");
            Keyboard.SendKeys(txtusrname, "XXXXX");

            //Passsowrd Textbox
            WpfEdit txtpasword = new WpfEdit(mainWindow);
            txtpasword.SearchProperties.Add(WpfButton.PropertyNames.AutomationId, "txtpassword");
            txtpasword.WindowTitles.Add("ValidatingControlsView");
            txtpasword.Text = "XXXXX";

            //Login button Click
            WpfButton blogin = new WpfButton(mainWindow);
            blogin.SearchProperties.Add(WpfButton.PropertyNames.AutomationId, "btnLogin");
            Playback.Wait(1000);
            Mouse.Click(blogin);}

Now Suppose i want to check all validation checks i.e both Positive ad negative what is best way to write them .i have tried below but i dont know it is the best way or not.

//validation checks
            string expectedUsername = "XXXX";
            string expectedPassword = "XXXXX";
            if (!string.IsNullOrEmpty(txtusrname.ToString()) && !string.IsNullOrEmpty(txtpasword.ToString()))
            {
                if (txtusrname.ToString() == expectedUsername && txtpasword.ToString() == expectedPassword)
                {
                    Assert.IsTrue(true, "Message.");
                }
                else { Assert.IsFalse(false, "Message"); }
            }
            else
            {
                Assert.IsFalse(false, "Message.");
            }