A small Unity helper class to convert viewport, screen or world positions to canvas space.

CanvasPositioningExtensions.cs

using UnityEngine;

/// <summary>
/// Small helper class to convert viewport, screen or world positions to canvas space.
/// Only works with screen space canvases.
/// </summary>
/// <example>
/// objectOnCanvasRectTransform.anchoredPosition = specificCanvas.WorldToCanvasPoint(worldspaceTransform.position);
/// </example>
public static class CanvasPositioningExtensions {
    public static Vector3 WorldToCanvasPosition(this UnityEngine.Canvas canvas, Vector3 worldPosition, Camera camera) {
        var viewportPosition = camera.WorldToViewportPoint(worldPosition);
        return canvas.ViewportToCanvasPosition(viewportPosition);
    }

    public static Vector3 ScreenToCanvasPosition(this UnityEngine.Canvas canvas, Vector2 screenPosition) {
        var viewportPosition = new Vector2(screenPosition.x / Screen.width, screenPosition.y / Screen.height);

        return canvas.ViewportToCanvasPosition(viewportPosition);
    }

    public static Vector3 ViewportToCanvasPosition(this UnityEngine.Canvas canvas, Vector2 viewportPosition) {
        //Gets the canvas's RectTransform
        //canvas.GetComponent<RectTransform>() would work too,
        //but since Transform of a UI Element is already a RectTransform
        //I prefer to use cast instead to avoid the overhead of a GetComponent
        var canvasTransform = (RectTransform) canvas.transform;

        //Viewport positions ranges from 0 to 1
        //Being (0, 0) the bottom left corner
        //and (1, 1) the top right corner of the viewport.
        //
        //The sizeDelta of a canvas is the same as the screen resolution it is currently on.
        //In a screen of 800x600, the canvas would have a sizeDelta of (x:800, y:600) too!
        //
        //With that information, we can "cast" a value from a viewport to its relative position on an (Unscaled) canvas
        //Vector2.Scale or Vector3.Scale does just that (by simple multiplying each axis from A with the same axis from B)
        var scaled = Vector2.Scale(viewportPosition, canvasTransform.sizeDelta);

        //If a CanvasScaler is being used, by just doing the above is not enough.
        //By scaling the result from above (again) with the scale of the canvas we can fix the position to account for canvas scaling
        return Vector2.Scale(scaled, canvasTransform.localScale);
    }
}