Unknown error and logic error in c# program of extension methods

Question

this is my c# code. There is an error I don't know, I am an intermediate programmer. check the code pls -

I have been programming and I didn't understand extension methods completely. I don't know much namespaces, and I know that my code is a bit stupid in the first class, but I put it here so you can check it out why it doesn't work. The main questions for me are -

1. Why is there an error in the program?
2. Why doesn't my namespace of ExtensionMethods work? I have implemented it at the top of the code also. If I make the program class non-static, and I use the namespace ExtensionMethods as normally to implement the methods, why doesn't work?

and um... here is the error details given to me by Visual Studio IDE -

System.InvalidOperationException
HResult=0x80131509
Message=Collection was modified; enumeration operation may not 
execute.
Source=mscorlib
StackTrace:
at 
System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource 
resource)
at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
at System.Collections.Generic.List`1.Enumerator.MoveNext()
at ConsoleApp1.Program.JoinWords(String s) in 

and this is the code -

    using System;
    using System.Diagnostics;
    using System.Collections.Generic;
    using System.Text;
    using ExtensionMethods;
    
    
    namespace ExtensionMethods
    {
        public static class TryExtensions
        {
            public static string JoinWords(this string s)
            {
                List<char> contents = new List<char>() { };
    
                foreach (char i in s)
                {
                    contents.Add(i);
                }
    
                foreach (char i in contents)
                {
                    if (i == ' ')
                    {
                        contents.Remove(i);
                    }
                }
    
                StringBuilder somestring = new StringBuilder();
    
                foreach (char i in contents)
                {
                    somestring.Append(i);
                }
    
                s = somestring.ToString();
    
                return s;
            }
        }
    }
    
    
    namespace ConsoleApp1
    {
        static class Program
        {
    
            public static string JoinWords(this string s)
            {
                List<char> contents = new List<char>() { };
    
                foreach (char i in s)
                {
                    contents.Add(i);
                }
    
                foreach (char i in contents)
                {
                    if (i == ' ')
                    {
                        contents.Remove(i);
                    }
                }
    
                StringBuilder somestring = new StringBuilder();
    
                foreach (char i in contents)
                {
                    somestring.Append(i);
                }
    
                s = somestring.ToString();
    
                return s;
            }
            static void Main(String[] args )
            {
                string sayHello = "Hello World, i really hope all spaces will be removed from this, and this sentence will be a joined one word";
    
                sayHello = JoinWords(sayHello);
    
                Debug.WriteLine(sayHello);
            }
        }
    }

right so this was the code

1. Why is there an error in the program?
2. Why doesn't my namespace of ExtensionMethods work? I have implemented it at the top of the code also.

Answer 1

An extension method is treated as if it was a method of the class (the string class in this case). So replace

sayHello = JoinWords(sayHello);

with

sayHello = sayHello.JoinWords();

In others words a string extension method should be treated exactly like any other string method.

You can remove JoinWords() from class Program.

Having said that, you have a runtime error caused by modifying contents inside the for each i in contents loop. You're not allowed to delete items in a list while you're iterating through that list with a foreach. The usual way to handle this is by saving the index values you want to delete in the foreach loop and then doing the remove in another loop.

Caution: Suppose you want to delete indexes i1, i2, i3. When you remove index i1, all following items are shifted left so you need to subtract 1 from i2 and i3. Then when you delete i2, you need to again subtract 1 from i3.