String to bool? How?

Question

Hello!
How can i fix my CS0019
{
/Playground/file0.cs(13,21): error CS0019: Operator '||' cannot be applied to operands of type 'bool' and 'string'
./Playground/file0.cs(21,21): error CS0019: Operator '||' cannot be applied to operands of type 'bool' and 'string'
./Playground/file0.cs(29,21): error CS0019: Operator '||' cannot be applied to operands of type 'bool' and 'string'
}
in this programm?
using System;

namespace SoloLearn
{
class Program
{
static void Settings(string a, string b, string c)
{
bool symbols;
bool topletter;
bool letter;
{
if (a == "Y" || "y" || "Yes" || "yes")
{
symbols = true;
}
else
{
symbols = false;
}
if (b == "Y" || "y" || "Yes" || "yes")
{
letter = true;
}
else
{
letter = false;
}
if (c == "Y" || "y" || "Yes" || "yes")
{
topletter = true;
}
else
{
topletter = false;
}
}
}
static void Main(string[] args)
{
string a, b, c;
Console.WriteLine("Please wright length of your pASSword:");
string lengthstring = Console.ReadLine();
int length = Int32.Parse(lengthstring);
Console.WriteLine("Do you need symbols?");
a = Console.ReadLine();
Console.WriteLine("Do you need Letters?");
b = Console.ReadLine();
Console.WriteLine("Do you need TOP Letters?");
c = Console.ReadLine();
Settings(a, b, c);
}
}
}

Answer 1

a=="y"||a=="Y"||a=="yes"||a=="Yes"