Trying to understand why my choice in an array doesn't work - c#-2.0

Question

Sorry for the ambiguous title. I'm kind of new to programming and this issue is a bit weird so I didn't know how to word it.
I'm simply practicing on creating an array populated with colors and the user just have to pick a color by picking 1-9. The issue is that I get an error message:
Use of unassigned local variable 'UserPickNum'
I already assigned it to an int and the only solution that will fix this if I created another construct, runs Console.WriteLine(colors[UserPickNum]); picks the color, then the program runs fine.
I guess my question is, why won't it work like this without having to create and call for another construct.
using System;
using System.Collections.Generic;
using System.Text;
class Program
{
static void Main()
{
string[] colors;
string Pick;
int PickNum;
int UserPickNum;
Console.WriteLine("Which color do you want? Pick 1-9");
Pick = Console.ReadLine();
if (int.TryParse(Pick, out PickNum))
{
UserPickNum = Convert.ToInt32(Pick);
}
else
{
Console.WriteLine("This isn't a valid number");
Console.ReadKey();
Main();
}
colors = new string[10] { "black", "white", "green", "purple", "red", "brown", "blue", "gray", "yellow", "indigo" };
Console.WriteLine(colors[UserPickNum]);
Main();
}

Answer 1

If Pick turns out not to be a number and thus int.TryParse returns false, then UserPickNum will not be assigned to a value. Probably the easiest way to resolve this is to assign a value to UserPickNum where you declare it, i.e.:
int UserPickNum = Int32.MinValue;

Answer 2

Your code only has one route through the if statement where you set UserPickNum - hence the warning. However, you don't need to call Convert.ToInt32 on Pick as you already have the number as an integer - in PickNum - assuming that the user entered a valid integer. This means you don't need UserPickNum at all. So you could change your code to be:
if (!int.TryParse(Pick, out PickNum))
{
Console.WriteLine("This isn't a valid number");
Console.ReadKey();
Main();
}
colors = new string[10] { "black", "white", "green", "purple", "red", "brown", "blue", "gray", "yellow", "indigo" };
Console.WriteLine(colors[PickNum]);
Main();
You should also check that it's in range before trying to use it as an array index:
if (PickNum < 0 || PickNum > 9)
{
Console.WriteLine("Input out of range");
// Try again
}

Answer 3

That error means that you are trying to use a variable that hasn't had a value given to it yet. You say "I already assigned it to an int", but that's not really an assignment. The assignment is when you give it a value (e.g. int UserPickNum = 0).
In your code it looks you're attempting to make sure the value is assigned with your if/else statement, but the else clause doesn't actually stop the rest of the code after it from executing. You would need to return after you recursively call Main().

Answer 4

The error message you are getting is because your variable UserPickNum isn't initialized (It isn't set in your else block, and thus there is potentially a path through your code where it isn't set to a value).
You can set it to a default value when you declare it to fix this problem:
int UserPickNum = 0;
Other Feedback
Something else I noticed is that you don't even need the UserPickNum variable. In this part of your code:
Pick = Console.ReadLine();
if (int.TryParse(Pick, out PickNum))
{
UserPickNum = Convert.ToInt32(Pick);
}
the Variable PickNum contains the value that the user typed in as an integer. You can streamline this part of the code like this:
Pick = Console.ReadLine();
if (!int.TryParse(Pick, out PickNum))
{
// code you are currently doing in your else block
Console.WriteLine("This isn't a valid number");
Console.ReadKey();
Main();
}
// code you are currently doing after your if else block
colors = new string[10] { "black", "white", "green", "purple", "red", "brown", "blue", "gray", "yellow", "indigo" };
Console.WriteLine(colors[PickNum]);
Main();