C# Thread object lifetime

Question

Suppose I have a code as follows:

int Main()
{
    if (true)
    {
       new Thread(()=>
          {
              doSomeLengthyOperation();
          })         


        

Answer 1

when does Thread A get destroyed?

When doSomeLengthyOperation finishes.

Will doSomeLenghtyOperation() be able to run into completion

Yes, even if the main thread exists because it is not a background thread. If you set the IsBackground property to true before starting the thread whenever the main thread exists, this thread will also be stopped.