Error 1 Cannot implicitly convert type 'int**' to 'int*'. An explicit conversion exists (are you missing a cast?)

Question

I'm learning C and C# and this question is for C#. I looking at pointers at msdn and this code is not compiling, it gives the error:Error 1 Cannot implicitly convert type int** to int*. An explicit conversion exists (are you missing a cast?). What am I missing here?

Here is the code:

    int ix = 10;
        unsafe
        {
            int* px1;
            int* px2 = &ix;  **The error is on this line**
        }

EDIT: Here is the program in its entirety:

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Management;
using System.Diagnostics;
using System.Windows.Forms;
using practice;

class Program
{
  public delegate bool ThisIsTheDelegate(int number);
  public delegate bool ThisIsAnotherDelegate(int number);

  private static string address;

  public static String Address
  {
      get
      {
          return address;
      }
      set
      {
          address = value;
      }
  }

    static void Main()
    {
        int[] someArray = new int[] { 1, 2, 3, 5, 6, 7, 8, 9, 0 };
        foreach (int number in someArray)
        {
            Console.WriteLine(number);
        }

        int integer = 98;
        string someString = "edited";
        string[] someStr = { "edited" };
        String[] anotherStringSomestr = new String[] { "edited" };

        var readWithLinq = from far in anotherStringSomestr
                           select far;

        foreach (var some in readWithLinq)
        {
            Console.WriteLine(some);
        }

        Program newPro = new Program();
        bool isEven1 = newPro.isEven(99);
        Console.WriteLine(isEven1);
        ThisIsTheDelegate newDelegate = newPro.isEven;
        Console.WriteLine(newDelegate(98));
        int[] numbers = { 1, 2, 3, 5, 6, 7, 8, 9 };
        List<int> evenNumbers = FilterArray(numbers, newDelegate);
        foreach(int integer1 in evenNumbers)
        {
            Console.WriteLine(integer1);
        }
        List<int> oddNumbers = FilterArray(numbers, isOdd);
        foreach (int integer1 in oddNumbers)
        {
            Console.WriteLine(integer1);
        }

        ThisIsAnotherDelegate anotherDelegate;
        anotherDelegate = number => (number % 2 == 0);
        Console.WriteLine("{0} is a even number", anotherDelegate(4));

        for (int i = 0; i < someString.Length; i++)
        {
            Console.WriteLine(someString);
        }

        for (int i = 0; i < someStr.Length; i++)
        {
            Console.WriteLine(someStr[i]);
        }

        Console.WriteLine(integer);

        SimpleStruct structss = new SimpleStruct();
        structss.DisplayX();

        M.x = 1;
        structss.x = 98;

        Console.WriteLine(M.x);
        Console.WriteLine(structss.x);

        M.structtaker(ref structss);
        M.classtaker();

        Console.WriteLine(structss.x);
        Console.WriteLine(M.x);

        M.x = 1;
        structss.x = 98;

        int ix = 10;
        unsafe
        {
            int* px1;
            int* px2 = &ix;
        }

        int selection = 98;
        while (selection != 0)
        {
            mainMenu();
            Console.Write("Enter choice: ");
            selection = Convert.ToInt32(Console.ReadLine());
            switch (selection)
            {
                case 0:
                    break;
                case 1:
                    openSomething();
                    break;
                case 2:
                    calculator();
                    break;
                case 3:
                    coolestProgramEverALive();
                    break;
                case 4:
                    make_to_do_list();
                    break;
                case 5:
                    add_to_do_list();
                    break;
                case 6:
                    readToDoList();
                    break;
                case 7:
                    linq_and_arrays();
                    break;
                case 8:
                    calendar();
                    break;
                case 9:
                    linq_and_collections();
                    break;
                default:
                    Console.WriteLine("Unkown selection. Try again");
                    break;
            }
        }
    }

    private static bool isOdd(int number)
    {
        return (number % 2 == 1);
    }

    private static List<int> FilterArray(int[] numbers, ThisIsTheDelegate newDelegate)
    {
        List<int> result = new List<int>();

        foreach (int item in numbers)
        {
            if (newDelegate(item))
                result.Add(item);
        }
        return result;
    }

    private static void linq_and_collections()
    {
        List<string> names = new List<string>();
        names.Add("Billy");
        names.Add("Steve");
        names.Add("Casandra");
        names.Insert(0, "Johanna");
        names.Add("Sonny");
        names.Add("Suzanne");
        names.Insert(2, "Sid");

        var queryLinqUpper = from name in names
                             where (name.StartsWith("S") || name.StartsWith("B") || name.StartsWith("J"))
                             let namesToUpper = name.ToUpper()
                             orderby namesToUpper
                             select namesToUpper;
        foreach (var linqToUpper in queryLinqUpper)
        {
            Console.Write(linqToUpper + " ");
        }
        Console.WriteLine();
        M.WriteTextToConsole("Hello, world. Programming in C# is fun");
        char c = 'A';
        int count = 14;
        String str = new String(c, count);
        str.WriteTextToConsole();
        M.WriteTextToConsole(str);

    }

    private static void calendar()
    {
        Application.Run(new Form1());
    }

    private static void readToDoList()
    {
        var files = from file in Directory.GetFiles(@"C:\data", "*.txt")
                    select file;
        int number = 1;
        foreach (var file in files)
        {
            Console.WriteLine(number + ". " + file);
            number++;
        }
        Console.Write("What todolist do you want to read? Give me the name:");
        try
        {
            string name = Console.ReadLine();
            Address = Path.Combine(@"C:\data", name + ".txt");

            TextReader inFile = new StreamReader(Address);
            while (inFile.Peek() != -1)
            {
                string line = inFile.ReadLine();
                Console.WriteLine(line);
            }
            inFile.Close();
        }
        catch (Exception ex)
        {
            MessageBox.Show("Exception thrown!", "Error");
            Console.WriteLine(ex.ToString());
        }
    }

    private static void linq_and_arrays()
    {
        int numberOfElements;
        Console.WriteLine("Start by setting the int[] array.");
        Console.Write("How many elements are there in your array?");
        numberOfElements = Convert.ToInt32(Console.ReadLine());
        int[] array = new int[numberOfElements];
        for (int i = 0, j = numberOfElements; i < numberOfElements; i++, j--)
        {
            Console.Write("Integers left to add {0}. Enter an integer:", j);
            array[i] = Convert.ToInt32(Console.ReadLine());
        }
        var arrayquery = from value in array
                         select value;

        foreach (var val in arrayquery)
        {
            Console.WriteLine("Value from array:{0}", val);
        }

    }

    private static void add_to_do_list()
    {
        Console.WriteLine("Which todolist do you want to modify?");
        listToDoLists();
        Console.Write("Enter name of todolist: ");
        string name = Console.ReadLine();
        Address = Path.Combine(@"C:\data", name + ".txt");
        String tempString;
        StreamWriter stream;
        stream = File.AppendText(Address);
        Console.Write("Enter your new ToDo: ");
        tempString = Console.ReadLine();
        stream.WriteLine(tempString);
        stream.Close();
        TextReader inFile;
        inFile = new StreamReader(Address);
        while (inFile.Peek() != -1)
        {
            string line = inFile.ReadLine();
            Console.WriteLine(line);
        }
        inFile.Close();
    }

    private static void listToDoLists()
    {
        int filenumber = 1;
        string[] filepaths = Directory.GetFiles("C:\\data\\", "*.txt");
        foreach (string file in filepaths)
        {
            Console.WriteLine(filenumber + ". " + file);
            filenumber++;
        }
    }

    private static void make_to_do_list()
    {
        string path, name;
        string yesOrNo;
        Console.Write("Enter name of todolist: ");
        name = Console.ReadLine();
        path = Path.Combine(@"C:\data", name + ".txt");
        TextWriter outFile = new StreamWriter(path);
        labelOne:  // else clause : unknown answer
        Console.WriteLine("Do you want to add something to todolist.Y/N?");
        yesOrNo = Console.ReadLine();
        if (yesOrNo.ToLower() == "y")
        {
            string line;
            int lines;
            Console.Write("How many lines?");
            lines = Convert.ToInt32(Console.ReadLine());
            for (int i = 0; i < lines; i++)
            {
                Console.Write("Enter a line of text: ");
                line = Console.ReadLine();
                outFile.WriteLine(line);
            }
            outFile.Close();
        }
        else if (yesOrNo.ToLower() == "n")
        {
            Console.WriteLine("You can close the application now.");
        }
        else
        {
            Console.WriteLine("Unknown answer. Try again");
            goto labelOne;
        }
    }

    private static void coolestProgramEverALive()
    {
        System.Diagnostics.Process.Start(@"C:\Users\KristjanBEstur\Documents\Visual Studio 2012\Projects\The_coolest_program_ever_alive\The_coolest_program_ever_alive\obj\Debug\The_coolest_program_ever_alive.exe");
    }

    private static void calculator()
    {
        System.Diagnostics.Process.Start("calc");
    }

    private static void openSomething()
    {
        System.Diagnostics.Process.Start("notepad");
    }

    private static void mainMenu()
    {
        Console.WriteLine("Main Menu");
        Console.WriteLine("0. Quit");
        Console.WriteLine("1. OpenSomething");
        Console.WriteLine("2. Calculator");
        Console.WriteLine("3. coolestProgramEverAlive");
        Console.WriteLine("4. Make todolist");
        Console.WriteLine("5. Add to todolist");
        Console.WriteLine("6. Read to do list");
        Console.WriteLine("7. Linq and arrays");
        Console.WriteLine("8. Calendar");
        Console.WriteLine("9. Linq and collections");
    }


    public bool isEven(int number)
    {
        return (number % 2 == 0);
    }
}

static class M
{
    public static int x;

  public static void WriteTextToConsole(this string text)
  {
      Console.WriteLine(text);
  }

  public static void structtaker(ref SimpleStruct s)
  {
      s.x = 5;
  }
  public static void classtaker()
  {
      M.x = 5;
  }
  }

 class Test
 {
   static int value = 20;
   unsafe static void F(out int* pi1, ref int* pi2) {
  int i = 10;
  pi1 = &i;
  fixed (int* pj = &value) {
     // ...
     pi2 = pj;
  }
    }
 }

struct SimpleStruct
{
public int x;
private int xval;
public int X
{
    get 
    {
        return xval;
    }
    set 
    {
        if (value < 100)
            xval = value;
    }
}
public void DisplayX()
{
    Console.WriteLine("The stored value is: {0}", X);
}

}

Answer 1

I've moved the expression "int i = 10;" out of the unsafe block and now it compiles. I've also put the code in question in a new project of another instance of vs2012pro. And that also compiles. So now I'm unable to replicate the error.

Here is the code:

        int ix = 10;
        unsafe
        {               
            int* px1;
            int* px2 = &ix;
            Test.F(out px1, ref px2);
            Console.WriteLine("*px1 = {0}, *px2 = {1}",
               *px1, *px2);   // undefined behavior
        }

Here is the other project:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace saxerium
{
class Program
{
    static void Main(string[] args)
    {
        int ix = 10;
        unsafe
        {
            int* px1;
            int* px2 = &ix;
            Test.F(out px1, ref px2);
            Console.WriteLine("*px1 = {0}, *px2 = {1}",
               *px1, *px2);   // undefined behavior
        }
    }
}

class Test
{
    static int value = 20;
    public unsafe static void F(out int* pi1, ref int* pi2)
    {
        int i = 10;
        pi1 = &i;
        fixed (int* pj = &value)
        {
            // ...
            pi2 = pj;
        }
    }
}

}

Answer 2

I wished I could replicate this it seems like a rather interesting issue. Here is the section of the standard that I believe applies to this (sorry about the formatting):

18.3 Fixed and moveable variables The address-of operator (§18.5.4) and the fixed statement (§18.6) divide variables into two categories: Fixed variables and moveable variables. Fixed variables reside in storage locations that are unaffected by operation of the garbage collector. (Examples of fixed variables include local variables, value parameters, and variables created by dereferencing pointers.) On the other hand, moveable variables reside in storage locations that are subject to relocation or disposal by the garbage collector. (Examples of moveable variables include fields in objects and elements of arrays.) The & operator (§18.5.4) permits the address of a fixed variable to be obtained without restrictions. However, because a moveable variable is subject to relocation or disposal by the garbage collector, the address of a moveable variable can only be obtained using a fixed statement (§18.6), and that address remains valid only for the duration of that fixed statement. In precise terms, a fixed variable is one of the following: • A variable resulting from a simple-name (§7.6.2) that refers to a local variable or a value parameter, unless the variable is captured by an anonymous function. • A variable resulting from a member-access (§7.6.4) of the form V.I, where V is a fixed variable of a struct-type. • A variable resulting from a pointer-indirection-expression (§18.5.1) of the form *P, a pointer-member-access (§18.5.2) of the form P->I, or a pointer-element-access (§18.5.3) of the form P[E]. All other variables are classified as moveable variables. Note that a static field is classified as a moveable variable. Also note that a ref or out parameter is classified as a moveable variable, even if the argument given for the parameter is a fixed variable. Finally, note that a variable produced by dereferencing a pointer is always classified as a fixed variable.

I spend some time trying to create a similar error but wasn't about to. The only way I was able to recreate the issue was with the following code:

    void test() {
        int ix = 10;
        unsafe {
            int* px1 = &ix;
            int* px2 = &px1; // **The error is on this line**
            }
        }

Of course this code cannot be fixed by moving the ix declaration into the safe scope. Perhaps you could try replicating the original problem in a very small bit of code (like above) and verify that both the problem and the fix replicate. Perhaps VS became confused. I have had problems that made no sense and went away by exiting VS and restarting (not often but a few times).

Answer 3

I moved the i declaration down inside the unsafe block and it fixed it, I don't know why?

Here is the code:

    unsafe
    {   
        int ix = 10;
        int* px1;
        int* px2 = &ix;
    }