How can I read user input from the console?

a = double.Parse(Console.ReadLine());

Beware that if the user enters something that cannot be parsed to a double, an exception will be thrown.

Edit:

To expand on my answer, the reason it's not working for you is that you are getting an input from the user in string format, and trying to put it directly into a double. You can't do that. You have to extract the double value from the string first.

If you'd like to perform some sort of error checking, simply do this:

if ( double.TryParse(Console.ReadLine(), out a) ) {
  Console.Writeline("Sonuç "+ a * Math.PI;); 
}
else {
  Console.WriteLine("Invalid number entered. Please enter number in format: #.#");
}

Thanks to Öyvind and abatischev for helping me refine my answer.

I think there are some compiler errors.

• Writeline should be WriteLine (capital 'L')

• missing semicolon at the end of a line

      double a, b;
      Console.WriteLine("istenen sayıyı sonuna .00 koyarak yaz");
      a = double.Parse(Console.ReadLine());
      b = a * Math.PI; // Missing colon!
      Console.WriteLine("Sonuç " + b);
  

string input = Console.ReadLine();
double d;
if (!Double.TryParse(input, out d))
    Console.WriteLine("Wrong input");
double r = d * Math.Pi;
Console.WriteLine(r);

---

The main reason of different input/output you're facing is that Console.Read() returns char code, not a number you typed! Learn how to use MSDN.

I'm not sure what your problem is (since you haven't told us), but I'm guessing at

a = Console.Read();

This will only read one character from your Console.

You can change your program to this. To make it more robust, accept more than 1 char input, and validate that the input is actually a number:

double a, b;
Console.WriteLine("istenen sayıyı sonuna .00 koyarak yaz");
if (double.TryParse(Console.ReadLine(), out a)) {
  b = a * Math.PI;
  Console.WriteLine("Sonuç " + b); 
} else {
  //user gave an illegal input. Handle it here.
}