Choose two different variable "collections" of listview items in C# on base of a condition for looping over them without two loops, how would I do that?

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C#2.0

C#

C#3.5

ListView

How would I pre-select a variable collection of listView items in C# WinForm with a different datatype for each, before I enter a loop with iterating through the items without doubling actually the loop itself for applying the condition?

bool SelectAll=false; //can be true or false it is a variable setting...
var myItems=null; //here first error, initialization is required of "var" and cannot be "null", so what to do? it must be an empty type to not preselect a type in var, else var cannot be changed anymore to another type.

/*here shall be the decision which collection of items shall 
be taken to loop through. Yes, I simply could put two loops with all this code, but that would suck to repeat all this. A function I cannot do, because the data type of the ListViewItemCollection and ListViewSelectedItemCollection is not the same and an object type is not allowed for enumeration (I tried it)!*/

if(SelectAll==true)
{
myItems=listView1.Items; //problem: this Listview Collection is a different data type as the other below!
}
else
{
myItems=listView1.SelectedItems; //problem: this Listview Collection is a different data type as the other above!
}

foreach(ListViewItem XYZ in myItems) //here shall be the loop not repeat through the condition, just one loop but the selection of the items to loop through could be different
{
//...many lines of codes inbetween
}

What I have tried:

tried 'var' datatype (needs to be initialized, cannot be initialized with null, if I initialize with the first data type I cannot later put another data type on it. it seems not to be enumerable and is not allowed to be returned in a function)

tried 'object' datatype (cannot be used as enumerable in the loop)

tried 'ListViewItemCollection' as ienumerable, but it cannot take another type like 'ListViewSelectedItemCollection'.

A second loop with all the code conditional repeated is what I do not want!
I tried a function, but the return datatype varies on a condition, it can be either 'ListViewItemCollection' or 'ListViewSelectedItemCollection'. So a function doesn't apply either.

Has someone another idea for variable items selection before the loop?