C#

private void KonverterBTN_Click(object sender, EventArgs e)// converter button code
{
    Celsiuslistbox.Items.Add(textBox1);
    
    double temp_Celsius; // temperature i celsius 
    double temp_Fahrenheit;// temperature i fahrenheit 
    int count;
    for ( count = 0; count <10; count++)
    {
        // you are copying the same value 10 times into Celsiuslistbox
        // and the return value from Items.Add is the item index, not the celsius value
        temp_Celsius = Celsiuslistbox.Items.Add(textBox1.Text);
    }
    
    do
    {
        // this do loop will run forever since count is equal to 10
        // from the for loop above
        temp_Celsius = Celsiuslistbox.Items.Add(textBox1.Text);
    } while ( count <=10);

    // this will convert the index number of the last item in the listbox to fahrenheit
    temp_Fahrenheit = (9.0 / 5.0) * temp_Celsius + 32;
       
    // this will not work since temp_Fahrenheit is a double, not a string
    FahrenheitListBox.Items.Add(temp_Fahrenheit);

public static class GenericMathExtensions
{
    public static double? Exp<T>(this T tvalue, double topower)
    where T : 
        struct, 
        IComparable, 
        IComparable<T>, 
        IConvertible, 
        IEquatable<T>, 
        IFormattable
    {
        if (typeof(T).IsNumeric())
        {
            try
            {
                return Math.Pow(Convert.ToDouble(tvalue), topower);
            }
            catch
            {
                 // throw invalid cast exception ?
            }
        }

        return null;
    }

    // based on:
    // https://stackoverflow.com/a/13179018/133321
    public static bool IsNumeric(this Type type)
    {
        switch (Type.GetTypeCode(type))
        {
            case TypeCode.Byte:
            case TypeCode.SByte:
            case TypeCode.UInt16:
            case TypeCode.UInt32:
            case TypeCode.UInt64:
            case TypeCode.Int16:
            case TypeCode.Int32:
            case TypeCode.Int64:
            case TypeCode.Decimal:
            case TypeCode.Double:
            case TypeCode.Single:
                return true;
            case TypeCode.Object:
                if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>))
                {
                    return Nullable.GetUnderlyingType(type).IsNumeric();
                    //return IsNumeric(Nullable.GetUnderlyingType(type));
                }
                return false;
            default:
                return false;
        }
    }
}