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list inside list. how to access items inside of it

I'm having some issues with my programm in c#.
Basically I have a list called mainList with 3 items in it. First two items are integers, but third one is another list containing more items.
mainList[0] = 8;
mainList[1] = 1;
mainList[2] = list;
By using foreach loop I'm able to print all of those items.
foreach (var i in (dynamic)(mainList[2]))
{
Console.WriteLine(i);
}
However I don't know how to access them. The thing is that I can't use indexes, because it is not an array. I would like to do something like this:
foreach (var i in (dynamic)(mainList[2]))
{
// First item is in datetime type, so I would like to change it to int
Console.WriteLine(Convert.ToInt64(i[1]));
}
Is there a way to access items inside list like we do it in arrays with a help of indexes?

Lists support the same index-based access as arrays, so you can use
mainList[n]
to access the nth entry in mainList.

I guess you are looking for something like this, although it is really unclear what you're asking:
foreach(var item in mainList)
{
if(item is System.Collections.IEnumerable)
{
foreach(var obj in ((System.Collections.IEnumerable)item))
{
Console.WriteLine(obj);
}
}
}

C# Winform (Entity Framework) - Iterate thru DBLocal

I am working with a project modeled on this link: Databinding with WinForms
On the form load on the link, the tutorial set the binding to:
this.categoryBindingSource.DataSource =
_context.Categories.Local.ToBindingList();
my question is, is it possible to iterate rows and columns on DBlocal? _context.Categories.Local? If Yes, how?
Thanks in Advance.

DbSet<T>.Local property is ObservableCollection<T> and you can simply use a for/foreach loop. You can use either of these options:
foreach (Category item in context.Category.Local)
{
//MessageBox.Show(item.Name);
}
for (int i = 0; i < context.Category.Local.Count; i++)
{
var item = context.Category.Local[i];
//MessageBox.Show(item.Name);
}
context.Category.Local.ToList()
.ForEach(item =>
{
//MessageBox.Show(item.Name);
});
Don't forget to first load data to Local.

Find item in list by property and know the item name

I have struggled with figuring out how to find an item in a list by checking for a property and then isolating and naming this item. I need to do this to set a value that corresponds with that specific string. It wouldn't work if I just found that there is one, or several items in the list that match the property. My code is massive, but here's a snippet of it:
List<string> diag_right = new List<string>();
diag_right.Add(b3); //Add the string b3
diag_right.Add(b5); //Add the string b5
diag_right.Add(b7); //Add the string b7
if (diag_right.Exists(a => a.Equals("")))
{
//Find the item (string name, b3, b5, etc.) that was found as matching the blank property
}
Is it possible to do this? I know I can do this by checking to see if each string matches this property individually, but is there a faster way to do this?

Many ways this can be achieved, I prefer simple approach, loop collection using index and modify matching field
for(int index=0; index< diag_right.Count(); index++)
{
if(diag_right[index] == "b3")
{
// update
diag_right[index] = "b8";
}
}
Linq approach
This approach I just used Linq to fetch all indexes for a matching string.
var indexes = diag_right.Where(e=>e.Equals("searchstring")).Select((c,i) => i).ToList();
foreach(int index in indexes) // loop through each line.
{
diag_right[index] = "newvalue"; // set new value.
}
Working Demo

Listbox Selected Item to array

I have a DataTable with 2 columns called "ID" and "Software" that I have used as a DataSource for a lst_Software multiselect listbox.
I'm trying to gather the ID for all the selected items in that have been selected and place that in an int[] array.
Listbox setup:
lst_Software.DataSource = software; //software is a DataTable
lst_Software.DisplayMember = "Software";
lst_Software.ValueMember = "ID";
I've tried below
List<int> list = new List<int>();
for (int i = 0; i < lst_Software.SelectedItems.Count; i++)
{
list.Add(Convert.ToInt32(lst_Software.SelectedValue.ToString()));
}
int[] software = list.ToArray();
I'm finding that I'm only getting the first selected value except it will not iterate through all... I know why though. I'm not using i to get passed through inside the for loop. I'm hoping someone can give me a direction to go to iterate through all the selected values.
Thank you

You're using lst_Software.SelectedValue.ToString() here in the loop so it only returns the one item. You have a for loop but you're not using the index variable. However, all of this is unnecessary really all you need is;
var items = lst_Software.SelectItems;
As that property is already the list of selected items. From there you can cast/convert them as you please.

SelectedValue is just the first selected value. You need to use SelectedItems.

dhtmlxtoolbar option list, How to get specific Id based on Text?

Is it possible in dhtmlx to do a reverse lookup on an option list, i.e. if I know the ItemText can I use it to lookup the Id?
i.e. something like this if such a function existed
var t = this.getAllListOptionText("tbOptionList").indexOf(Name);
I want to get the id, so that I can preset the selection to a specific option.
I could potentially loop through all the options and look for the ItemText myself, however if something already exists that would be more elegant.
for (var i = 0; i < lsTags.length; i++) {
if (this.getListOptionText("tbOptionList", lsTags[i]) == Name) {
lOptionID = lsTags[i];
break;
}
}

There are dhtmlx methods forEachItem and forEachListOption, you can iterate items and use getItemText with a check.