oop_refactoring.cs

// before refactoring:
public class CalculatorForm {
    private string aText, bText;
    
    private bool IsValidInput(string text) => true;
    
    private void btnAddClick(object sender, EventArgs e) {
        if ( !IsValidInput(bText) || !IsValidInput(aText) ) {
            return;
        }
    }
}


// after refactoring:
public class CalculatorForm {
    private string aText, bText;
    
    private readonly IInputValidator _inputValidator;
    
    public CalculatorForm(IInputValidator inputValidator) {
        _inputValidator = inputValidator;
    }
    
    private void btnAddClick(object sender, EventArgs e) {
        if ( !_inputValidator.IsValidInput(bText)
            || !_inputValidator.IsValidInput(aText) ) {
            return;
        }
    }
}

public interface IInputValidator {
    bool IsValidInput(string text);
}

public class InputValidator : IInputValidator {
    public bool IsValidInput(string text) => true;
}

public class InputValidatorFactory {
    public IInputValidator CreateInputValidator() => new InputValidator();
}