If we don’t use an IDisposable in a using block, will the Dispose method be called?

The answer is No. The Dispose method is invoked by the runtime only when the class that implements the IDisposable interface is initialized with a static block setup.

class MyClass : IDisposable 
{
  public void Dispose()
  {
     Console.WriteLine("Calling MyClass.Dispose");
  }
}

MyClass c = new MyClass();
using(MyClass uc = new MyClass()) {}

output:
Calling MyClass.Dispose  //printed only once