Why am I getting an exception InvalidOperationException?

Question

I have written the following code which removes the selected value from the listBox. I also want to remove it from the Dictionary List and then update/write it to the text file so when I run the program again and load the text file it will be updated if not it will keep showing the removed item each time I run my application again.

private void listBox1_KeyDown(object sender, KeyEventArgs e)
        {
            string sb;
            if (e.KeyCode == Keys.Delete)
            {
                if (this.listBox1.SelectedIndex >= 0)
                {
                    string obj = this.listBox1.SelectedValue.ToString();
                    data.Remove(obj);
                    listBox1.DataSource = null;
                    listBox1.DataSource = data;
                    foreach (KeyValuePair<string, List<string>> kvp in LocalyKeyWords)
                    {
                        for (int i = 0; i < kvp.Value.Count(); i++)
                        {
                            sb = "Url: " + kvp.Key + " --- " + "Local KeyWord: " + kvp.Value[i] + Environment.NewLine;
                            LocalyKeyWords.Remove(kvp.Key);
                        }
                    }

                }
            }

        }

LocalyKeyWords is a Dictionary>

In this case it contain two items/keys I remove one and I see with a breakpoint that this one have been removed.

The question is if I need to remove the kvp.Key or to remove somehow the item I removed from the listBox wich is the one I want to remove from the LocalyKeywords and it's the obj variable since im doing:

data.Remove(obj);

So maybe I need to remove obj also from the localyKeyWords ?

The excpetion the error im getting is after it's removing the item from the LocalyKeyWords an click continue on this line:

foreach (KeyValuePair<string, List<string>> kvp in LocalyKeyWords)

Im getting the error:

Collection was modified; enumeration operation may not execute

System.InvalidOperationException was unhandled
  HResult=-2146233079
  Message=Collection was modified; enumeration operation may not execute.
  Source=mscorlib
  StackTrace:
       at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
       at System.Collections.Generic.Dictionary`2.Enumerator.MoveNext()
       at GatherLinks.Form1.listBox1_KeyDown(Object sender, KeyEventArgs e) in d:\C-Sharp\GatherLinks\GatherLinks\GatherLinks\Form1.cs:line 959
       at System.Windows.Forms.Control.OnKeyDown(KeyEventArgs e)
       at System.Windows.Forms.Control.ProcessKeyEventArgs(Message& m)
       at System.Windows.Forms.Control.ProcessKeyMessage(Message& m)
       at System.Windows.Forms.Control.WmKeyChar(Message& m)
       at System.Windows.Forms.Control.WndProc(Message& m)
       at System.Windows.Forms.ListBox.WndProc(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
       at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
       at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
       at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.Run(Form mainForm)
       at GatherLinks.Program.Main() in d:\C-Sharp\GatherLinks\GatherLinks\GatherLinks\Program.cs:line 18
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

Answer 1

You are removing items from LocalyKeyWords while in the midst of iterating over it; that is not allowed, as the exception message says.

I 'm not sure what the big picture is here, but a localized solution would be to make a temporary copy of LocalyKeyWords and iterate over that. You can then modify the "source" collection without any trouble.

Example:

foreach (var kvp in LocalyKeyWords.ToList())  // .ToList() makes a temp copy

Answer 2

You can't modify the IEnumerable collection inside of foreach.

You can create a copy of the dictionary and loop through it modifing the original one.