How do I fix this ArgumentNullException in int.Par

Question

This is the .cs file runs fine in Mono:

using System;

public class HelloWorld
{
    static public void Main ()
    {
    Console.WriteLine("Enter a number");

    int UserNumber = int.Parse(Console.ReadLine());

    Console.WriteLine("Your number is: " + UserNumber);
    }
}

I opened this Test.cs file in Xamarin, which worked properly. Then I choose 'Run' > 'Start Without Debugging' and these errors pop up in the display panel:

Enter a number

Unhandled Exception:
System.ArgumentNullException: Argument cannot be null.
Parameter name: String
  at System.Number.StringToNumber (System.String str, NumberStyles options, System.NumberBuffer& number, System.Globalization.NumberFormatInfo info, Boolean parseDecimal) [0x00054] in /private/tmp/source-mono-mac-4.0.0-branch-c5sr2/bockbuild-mono-4.0.0-branch/profiles/mono-mac-xamarin/build-root/mono-4.0.2/external/referencesource/mscorlib/system/number.cs:1084 
  at System.Number.ParseInt32 (System.String s, NumberStyles style, System.Globalization.NumberFormatInfo info) [0x00014] in /private/tmp/source-mono-mac-4.0.0-branch-c5sr2/bockbuild-mono-4.0.0-branch/profiles/mono-mac-xamarin/build-root/mono-4.0.2/external/referencesource/mscorlib/system/number.cs:755 
  at System.Int32.Parse (System.String s) [0x00000] in /private/tmp/source-mono-mac-4.0.0-branch-c5sr2/bockbuild-mono-4.0.0-branch/profiles/mono-mac-xamarin/build-root/mono-4.0.2/external/referencesource/mscorlib/system/int32.cs:140 
  at HelloWorld.Main () [0x0000b] in /Users/Yardenbourg/Desktop/Test.cs:9 
[ERROR] FATAL UNHANDLED EXCEPTION: System.ArgumentNullException: Argument cannot be null.
Parameter name: String
  at System.Number.StringToNumber (System.String str, NumberStyles options, System.NumberBuffer& number, System.Globalization.NumberFormatInfo info, Boolean parseDecimal) [0x00054] in /private/tmp/source-mono-mac-4.0.0-branch-c5sr2/bockbuild-mono-4.0.0-branch/profiles/mono-mac-xamarin/build-root/mono-4.0.2/external/referencesource/mscorlib/system/number.cs:1084 
  at System.Number.ParseInt32 (System.String s, NumberStyles style, System.Globalization.NumberFormatInfo info) [0x00014] in /private/tmp/source-mono-mac-4.0.0-branch-c5sr2/bockbuild-mono-4.0.0-branch/profiles/mono-mac-xamarin/build-root/mono-4.0.2/external/referencesource/mscorlib/system/number.cs:755 
  at System.Int32.Parse (System.String s) [0x00000] in /private/tmp/source-mono-mac-4.0.0-branch-c5sr2/bockbuild-mono-4.0.0-branch/profiles/mono-mac-xamarin/build-root/mono-4.0.2/external/referencesource/mscorlib/system/int32.cs:140 
  at HelloWorld.Main () [0x0000b] in /Users/Yardenbourg/Desktop/Test.cs:9 
The application was terminated by a signal: SIGHUP

I am not sure what the problem here is. Could it be to do with this line?

int UserNumber = int.Parse(Console.ReadLine());

Answer 1

I bet you're using a C# -> Mac -> Xamarin.Mac project. By default, these programs don't use an interactive console, which you're trying to use when you call Console.ReadLine().

Try creating a new solution; pick C# -> Console Project instead to have the interactive console working.

Answer 2

Read the stack trace, it says the method of Parse was passed a parameter of null, but it cannot be null. Try splitting the read line and the parsing, and then making sure the line is not null or empty.

public class HelloWorld
{
    static public void Main ()
    {
    Console.WriteLine("Enter a number");
    String input = Console.ReadLine();
    int UserNumber = 0;
    if(input != null && input != "")
    {
        UserNumber = int.Parse(input);
    }

    Console.WriteLine("Your number is: " + UserNumber);
    }
}

Splitting up code like this makes it easier to read and easier to debug.