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if (obj is string str)
   {
   ...
   }

for(int ic=list.Count,i=0;i<ic;++i) // ...

for(i = 0; i < list.Count; i++)

int itemsCount=list.Count;
for(i = 0; i < itemsCount; i++)

00007FF8646F1807  xor         edx,edx
00007FF8646F1809  mov         r8d,dword ptr [rcx+18h] ; this gets the Count from the List, once.
00007FF8646F180D  test        r8d,r8d
00007FF8646F1810  jle         00007FF8646F1830 ; skip loop if count is zero
; loop goes back here
00007FF8646E1812  cmp         edx,r8d          ; weird redundant loop end check
00007FF8646E1815  jae         00007FF8646E183A
00007FF8646E1817  mov         r9,qword ptr [rcx+8]  ; get array out of list
00007FF8646E181B  cmp         edx,dword ptr [r9+8]  ; get length of array/compare
00007FF8646E181F  jae         00007FF8646E1863      ; do bounds check :(
00007FF8646E1821  movsxd      r10,edx  ; unnecessary because the index is non-negative
00007FF8646E1824  add         eax,dword ptr [r9+r10*4+10h]
00007FF8646E1829  inc         edx
00007FF8646E182B  cmp         edx,r8d
00007FF8646E182E  jl          00007FF8646E1812

if (obj is string str)
   {

private void AnEventHandler(object sender, EventArgs e)
   {
   if (sender is TextBox tb)
      {
      ...
      }
   }

private void AnEventHandler(object sender, EventArgs e)
   {
   if (sender is TextBox)
      {
      TextBox tb = (TextBox) tb;
      ...
      }
   }

private void AnEventHandler(object sender, EventArgs e)
   {
   TextBox tb = sender as TextBox;
   if (tb != null)
      {
      ...
      }
   }

private void MyMethod()
    {
        {
        if (System.Int32.TryParse("666", out int i))
            {
            Console.WriteLine(i); // OK Here
            }
        Console.WriteLine(i);     // Annoyingly, OK Here
        }
    Console.WriteLine(i);         // Correctly, not OK here.
    }