Convert array of string to array of double

You can use Linq:

  Double fi[] = File.ReadLines("list of stopword.txt")
                    .Select(x => Double.Parse(x, CultureInfo.InvariantCulture))
                    .ToArray();

You get the error because you are trying to assign a double(Convert.ToDouble) to a double[] variable. Instead you have to parse every line to double and create an array.

You can use LINQ:

double value = 0;
double[] f1 = File.ReadLines("decimal list.txt")
    .Where(l => double.TryParse(l.Trim(), out value))  // skips invalid lines
    .Select(l => value)
    .ToArray();

The only Convert.ToDouble overload that matches is the one that receives Object. This function, like all the other ToDouble overloads, returns double.

public static double ToDouble(
    Object value
)

You are therefore attempting to assign a double to a double[] and the compiler tells you that it cannot do so.

Of course, if you ever did pass your string[] array to that Convert.ToDouble overload it would fail with a runtime error because Convert.ToDouble expects to receive a single value, and return a single value. It simply does not convert multiple values in a single call.

What you need to do is convert each line from the file into a double. An effective way to do that would be to use a Linq query as demonstrated by various other answers.

You could loop through the string array and use Double.Parse or Double.TryParse (if you might have strings that won't parse as doubles in the array).

Something like (not tested):

string[] lines = File.ReadAllLines("decimal list.txt");
List<doulbe> values = new List<double>();

foreach(string line in lines)
{
    double value;

    if (double.TryParse(line, out value))
    {
        values.Add(value);
    }
}

That's because Convert.ToDouble returns a single double-precision value and you have double[] (array of double-precision variables) on the left side.

Try instead:

List<double> list = new List<double>();
foreach(string line in File.ReadLines("list of stopword.txt"))
  list.Add(double.Parse(line));