Why is uninitialized GameObject != null?

I have an script object with references:

 public GameObject prefabObject;
 private GameObject instanceObject;

Which is self-explanatory;

Then I want to show and hide it this way:

 private void ShowObject()
 {
     if (this.instanceObject = null)
     {
         this.instanceObject = (GameObject)Instantiate(this.prefabObject);
     }
 }
 
 private void HideObject()
 {
     if (this.instanceObject != null)
     {
         Destroy(this.instanceObject);
     }
 }

But when trying to instantiate it, it never comes under "if null" check, like instanceObject is never null. Why is that?