✅ How to set a classname based off view model boolean avalonia

<TextBlock Classes="{Binding IsError ? Error : NewUserInput}" ... />

<TextBlock Classes="{Binding IsError ? Error : NewUserInput}" ... />

public CreateNewUserViewModel()
{
    private bool _isError = false;

    public UserModel ReturnUser()
    {
        bool usernameValid = ValidateUsername(Username);
        bool createPasswordValid = ValidatePassword(CreatePassword);
        bool confirmPasswordValid = ValidatePassword(ConfirmPassword);
        bool passwordsMatch = CreatePassword == ConfirmPassword;

        if (!usernameValid)
        {
            IsError = true;
            ErrorText = "...";
        }
    }
}

public CreateNewUserViewModel()
{
    private bool _isError = false;

    public UserModel ReturnUser()
    {
        bool usernameValid = ValidateUsername(Username);
        bool createPasswordValid = ValidatePassword(CreatePassword);
        bool confirmPasswordValid = ValidatePassword(ConfirmPassword);
        bool passwordsMatch = CreatePassword == ConfirmPassword;

        if (!usernameValid)
        {
            IsError = true;
            ErrorText = "...";
        }
    }
}