How would i convert this from JS into C#?

pretty simple code but i have no idea how to make a c# version. i tried making vector 3s for everything but that seemed way too inefficient.

var screenBoundary: float;

if(transform.position.x < -screenBoundary)
transform.position.x = -screenBoundary;
(transform.position.x > screenBoundary)
transform.position.x = screenBoundary;

Easy one!

float screenBoundary;
 
if(transform.position.x < -screenBoundary)
transform.position.x = -screenBoundary;
(transform.position.x > screenBoundary)
transform.position.x = screenBoundary;

Simple

float screenBoundary;

// you need to store position on a local variable
var position = transform.position;

// then you can modify its fields
if(position.x < - screenBoundary)
	position.x = -screenBoundary;
if(position.x > screenBoundary)
	position.x = screenBoundary;

// and then deploy to the original variable
transform.position = position;

Fun fact:

your last code would actually work, but you was missing some .x and parentheses

if ((transform.position = new Vector3(transform.position.x, transform.position.y, transform.position.z)).x < -screenBoundary)
{
	transform.position = new Vector3(-screenBoundary, transform.position.y, transform.position.z);
}
if ((transform.position = new Vector3(transform.position.x, transform.position.y, transform.position.z)).x > screenBoundary)
{
	transform.position = new Vector3(screenBoundary, transform.position.y, transform.position.z);
}

anyway those attributions inside the if conditions aren’t necessaries so you can simplify your code to look like this:

if (transform.position.x < -screenBoundary)
{
	transform.position = new Vector3(-screenBoundary, transform.position.y, transform.position.z);
}
if (transform.position.x > screenBoundary)
{
	transform.position = new Vector3(screenBoundary, transform.position.y, transform.position.z);
}