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To convert a string to a number in C#, you can use the built-in methods provided by the Convert class:

1. Use the Convert.ToInt32 method to convert a string to an integer:

   string str = "123";
   int number = Convert.ToInt32(str);
   

2. Use the Convert.ToDouble method to convert a string to a double:

   string str = "123.45";
   double number = Convert.ToDouble(str);
   

3. Use the int.TryParse or double.TryParse methods to convert a string to an integer or double, respectively. These methods return a boolean value indicating whether the conversion was successful or not:

   string str = "123";
   int number;
   if (int.TryParse(str, out number))
   {
      // Conversion successful
   }
   
   string str = "123.45";
   double number;
   if (double.TryParse(str, out number))
   {
      // Conversion successful
   }
   

Note that if the string cannot be converted to a number, an exception will be thrown in the first two methods. The TryParse methods are safer since they will not throw an exception and instead return a boolean value indicating the success of the conversion.