Exception Handling with Multiple Variables - if-statement

Question

I'm trying to learn more about exception handling while working on my program. I have multiple test variables I want to test and make sure it is within range with:
public bool IsWithinRange(TextBox textbox, string name, int min, int max)
{
double number = double.Parse(textbox.Text);
if (number < min || number > max)
{
MessageBox.Show(name + " must be between " + min.ToString() + " and " + max.ToString() + ".", "Entry Error");
textbox.Focus();
return false;
}
else { return true; }
}
And calling the method using:
bool condition;
condition = CheckAll();
if (condition == true) { condition = IsWithinRange(txtVar1, "Var1", 1, 50); }
if (condition == true) { condition = IsWithinRange(txtVar2, "Var2", -100, 100); }
if (condition == true) { condition = IsWithinRange(txtVar3, "Var3", 100, 200); }
This logic works, but I was curious to see if there was a more concise, better looking way of writing some form of systematic checking of variables one by one?

Answer 1

You can take advantage of a few things:
Are you able to assign meaningful names to the TextBox.Name properties? If so, you can omit the second parameter in "IsWithinRange" and simply call "Textbox.Name".
As of C# 6.0, there is now a syntax to interpolate strings. So the string passed into your your MessageBox.Show syntax can be made shorter and prettier.
You can immediately assign to "condition", and you can convert your "if" statements to combined "and" statements.
All together, your code can look like this:
bool condition =
CheckAll()
&& IsWithinRange(txtVar1, 1, 50)
&& IsWithinRange(txtVar2, -100, 100)
&& IsWithinRange(txtVar3, 100, 200);
// Some other code here
With your method looking like this:
public bool IsWithinRange(TextBox textbox, int min, int max) {
double number = double.Parse(textbox.Text);
if (number < min || number > max) {
MessageBox.Show($"{textbox.Name} must be between {min} and {max}.", "Entry Error");
textbox.Focus();
return false;
}
else
return true;
}
This is assuming you actually use "condition". If not, you can omit "bool condition = " and the code runs just the same.
But there are a few things to note. Your code will continue to run even if "CheckAll" is false or any "IsWithinRange" is false. This is true in my version above or in your own version. Yes, your user will get a message, but after he clicks "okay", the remaining code will run even if the checks fail.
Also, "IsWithinRange" might be misinterpreted by a teammate or even by yourself in the future. This is because it does more than just return true/false: it sends a message if false. This violates the principle of command-query separation.
An approach to these issues ignores brevity, as that is desired but never the highest goal. What you can do is create a class that validates, whose methods separate the tasks:
class Validator {
public bool isValid = true;
public List<string> messages = new List<string>();
public Validator CheckAll() {
// Whatever your logic is for this.
return this; // Return the instance of "Validator" that called this method
}
public Validator CheckRange (TextBox textbox, int min, int max) {
double number = double.Parse(textbox.Text);
if (number < min || number > max) {
messages.Add($"{textbox.Name} must be between {min} and {max}.");
isValid = false;
}
return this;
}
public void ShowErrorsToUser () =>
MessageBox.Show(string.Join(Environment.NewLine, messages));
}
Which you would use like this:
var validator =
new Validator()
.CheckAll()
.CheckRange(txtVar1, 1, 50)
.CheckRange(txtVar2, -100, 100)
.CheckRange(txtVar3, 100, 200);
if (!validator.isValid) {
validator.ShowErrorsToUser();
txtVar1.Focus();
return; // Stop code execution!
}
// Continue with your normal logic that utilizes your textbox values.
I'll leave it to you to decide whether the class-based approach is worth your time. But I present it to you as a different way to think.