Binary search with recursion c#

Question

I got stuck with my binary search method and I don't really know what I'm doing.

At this moment I got

1 Error (CS0161) not all code paths return a value

int low = 0;
int high = cities.Length;

static int BinarySearch(City[] cities, int key, int low, int high)
{
    int mid;

    if (low > high)
        return -1; //Sökning EJ hittad

    mid = low + high / 2;

    if (key == cities[mid].temp)
        return mid; //Sökning Hittad!
    else if (key < cities[mid].temp)
        BinarySearch(cities, key, low, mid - 1);
    else
        BinarySearch(cities, key, mid + 1, high);
}

...searching for temperature with input from user.

Console.WriteLine("\n\tBINARY SEARCH\n");
do
{
    Console.Write("Search temp:");
    loop = true;
    str = Console.ReadLine();
    try
    {
        key = Convert.ToInt32(str);
        
         
        index = BinarySearch(cities, key, low, high);
        if (index == -1)
        {
            Console.WriteLine($"Couldn't find any city with temperature: {key}°C");
        }
        else
        {
            Console.WriteLine("Search results: ");
            Console.WriteLine(cities[index].ToString());
            loop= false;
        }
    }
    catch
    {
        Console.WriteLine("Only numbers, please.");
    }
} while (loop);

Answer 1

The error message 'not all code paths return a value' is correct:

if (key == cities[mid].temp)
    return mid; //Sökning Hittad!
else if (key < cities[mid].temp)
    BinarySearch(cities, key, low, mid - 1); // <-- does not return anything
else
    BinarySearch(cities, key, mid + 1, high); // <-- does not return anything

Fix:

if (key == cities[mid].temp)
    return mid; //Sökning Hittad!
else if (key < cities[mid].temp)
    return BinarySearch(cities, key, low, mid - 1); 
else
    return BinarySearch(cities, key, mid + 1, high);