Why can I assign doubles sometimes without value?

I wrote a small WF program which caculates some condensators and the ohmic law. I now want to tidy up a little bit. I ran across a issue where I use 2 doubles which both got assigned the value 0. I can remove the value from the voltage double. But not the current one. And I can’t figure out why. Is there anything I am missing? Error Message is CS0165 Use of unassigned local variable ‘current’ and occurs in the line where the CalcResistance Method gets called

using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Kondensator_Ohmsches_Gesetz_Calc
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {       
          
        }

        private void calcResistance_Click(object sender, EventArgs e)
        {
            double voltage;
            double current;
            bool ok = double.TryParse(textBox1.Text, out voltage) && double.TryParse(textBox2.Text, out current);
            if (ok)
            {
                textBox3.Text = Formulacollection.CalcResistance(voltage, current);
            }
            else
            {
                textBox3.Text = "Error Format not found";
            }
            textBox4.Text = Formulacollection.ConvertMicro(textBox3.Text);
            textBox5.Text = Formulacollection.ConvertMilli(textBox3.Text);
            textBox6.Text = Formulacollection.ConvertKilo(textBox3.Text);
            textBox7.Text = Formulacollection.ConvertMega(textBox3.Text);

        }
}

Formulacollection Class works like this:

using System;
using System.Collections.Generic;
using System.Text;
using static System.Math;
using System.Numerics;
using System.Globalization;

namespace Kondensator_Ohmsches_Gesetz_Calc
{
    public static class Formulacollection
    {
        public static string CalcResistance(double voltage, double current)
        {
            var resistance = voltage / current;

            return resistance.ToString();
        }
}

>Solution :

The second part won’t always be evaluated when you use the && operator. Try to use & instead of it, or set a default value to current when you declare it.