BlockPlacer.cs

using UnityEngine;
using UnityEngine;
using System.Collections;

public class BlockPlacer : MonoBehaviour {

    //Handles # of blocks the user has
    public bool unlimitedBlocks = false;
    public int blocksLeft = 0;

    //If the player wants to lock the cursor somewhere else instead, they can do that.
    public bool lockCursor = true;

    //static reference to the script. This is so blocks can be added via a static method.
    static BlockPlacer obj;

    //The block prefab to instantiate
    public GameObject blockPrefab;

    //Maximum range that the player can place a block from
    public float range = 7f;

    int layerMask = 1 << 8;
    void Start () {
        //This assigns the static reference
        BlockPlacer.obj = this;
        layerMask = ~layerMask;
    }
    void Update () {
        //Locks the cursor. Running this in update is only done because of weird unity editor shenanigans with cursor locking
        if (lockCursor) {
            // ERRORS? Are you getting errors on the following line of code?
            // If so, you're probably using Unity 4 or earlier.
            // To fix this, add
            // to the start of the following line
            Cursor.lockState = CursorLockMode.Locked;
        }
        // And remove
        // from the start of the next line of code
        //Cursor.lockCursor = true;
        //Make sure the player has enough blocks
        if (blocksLeft > 0 || unlimitedBlocks) {
            //Place blocks using the LMB
            if (Input.GetMouseButtonDown(0)) {
                //Make a ray and raycasthit for a raycast
                Ray ray = new Ray(transform.position, transform.TransformDirection(Vector3.forward));
                RaycastHit hit;

                //Perform the raycast
                if (Physics.Raycast(ray, out hit, range, layerMask)) {
                    //The raycast is backed up so that placing works and won't place blocks inside of the ground.
                    //After testing, 0.2 units back had the best result
                    Vector3 backup = ray.GetPoint(hit.distance - 0.2f);

                    //Round the placement so they place like blocks should
                    Vector3 placeAt = new Vector3(
                        Mathf.RoundToInt(backup.x), Mathf.RoundToInt(backup.y), Mathf.RoundToInt(backup.z));

                    //Instantiate the block and save it so that we can do other stuff with it later
                    GameObject block = (GameObject)GameObject.Instantiate(blockPrefab, placeAt, Quaternion.Euler(Vector3.zero));

                    //Remove a block from the player's "inventory"
                    if (!unlimitedBlocks) {
                        blocksLeft--;
                    }

                    //If the block collides with the player, remove it. We don't want the player to get stuck in their own blocks.
                    if (block.GetComponent<Collider>().bounds.Intersects(this.transform.parent.GetComponent<Collider>().bounds)) {
                        //If they are, destroy the block
                        GameObject.Destroy(block);
                        //Make sure that the player gets their misplaced block back.
                        if (!unlimitedBlocks) {
                            blocksLeft++;
                        }
                    }
                }
            }
        }
    }

    void OnGUI() {
        //This is the crosshair
        GUI.Box(new Rect(Screen.width / 2 - 5, Screen.height / 2 - 5, 5, 5), "");
    }

    //Static adding of blocks. This isn't needed but definitely helps a lot
    public static void addBlocks(int i = 1) {
        BlockPlacer.obj.blocksLeft += i;
    }
}