Drag an object in unity3d

You can do this using plain mouseup/down tests and raycasting, and the same pattern applies to many other situations, like marquee selection.

// Call this inside an Update method.
void HandleDragging()
{
    const int mouseButton = 0; // button values are 0=left,1=right,2=middle
    if(_isDragging) // Leading underscore denotes private member variables
    {
        if(Input.GetMouseButton(mouseButton)) 
            UpdateDrag(); // Move your object according to Input.mousePosition
        if(Input.GetMouseButtonUp(mouseButton))
        {
            _isDragging = false;
            EndDrag(); // Set the object's position to its final destination
        }
    }
    else if (Input.GetMouseButtonDown(mouseButton))
    {
        // The bottom-left of the screen or window is at x:0, y:0
        var ray = Camera.main.ScreenPointToRay(Input.mousePosition);
        RaycastHit hit;
        // This requires a Collider component on the draggable objects
        if (Physics.Raycast(ray, out hit))
        {
            // Here you can filter down the kind of objects/components
            //     I've used gameObject, but transform is what you'll
            //     modify to move the object around.
            var dragObject = hit.transform.gameObject;
            _isDragging = true;
            // Here you can capture the object being dragged,
            //     or create a phantom representing the destination
            //     until the drag is completed or cancelled.
            BeginDrag(dragObject);
        }
    }
}

When you start drag (OnMouseDown), save object position in some variable. Then OnMouseUp you can test that object is in position you needed else restore object position by saved variable.

UPD:

// Define some target to place object in. It can be any Collider
public Collider target;
// Variable for saving initial object position
private Vector3 initialPosition;

void OnMouseDown()
{
    drag = true;
    // saving initial object position
    initialPosition = transform.position;
    //Debug.Log("haii ");
}

void OnMouseDrag()
{
    if(drag)
    {
        DragMe();
    }
}

void OnMouseUp()
{
    // UPD2: missed some test if object are actually dragging now
    if(drag) {
        // Testing object and target collision
       if(TestObjectInPlace(target, transform))
        {
            // Place object in target position if they are colliding
            transform.position = position;
            // or you can call DragMe() to put object under mouse cursor
            // DragMe();
        } else {
            // Restore initial object position if object is not collide with target
            transform.position = initialPosition;
        }
        drag = false;
    }
}

void DragMe()
{
    var pos =Input.mousePosition;
    pos.x =Mathf.Clamp(pos.x,0,Screen.width);
    pos.y =Mathf.Clamp(pos.y,0,Screen.height);
    if(zdist>0)
    {
        zdist=Vector3.Distance(Camera.main.transform.position,transform.position);
        pos.z=zdist;
        pos = Camera.main.ScreenToWorldPoint(pos);
        transform.position =pos;
    }
}

bool TestObjectInPlace(Collider target, Transform obj)
{
    // I test some distance between object and target for demo 
    // (I use distance less than 1 unit, but you can use any parameter of Collider
    // to test collision of target and object and if they are colliding return true 
    // or false if not)
    return Vector3.Distance(target.transform.position, obj.position) < 1;
}