C# - RectangleF will not Offset when used as Property

[MSwaffer]

When I run the following tests (In NUnit):

    [Test]
    public void ShouldOffsetRectangle() {
      RectangleF standAloneRect = new RectangleF(0, 0, 10, 10);
      standAloneRect.Offset(1, 1);
      Assert.AreEqual(1, standAloneRect.X, "standAlone failed");
    }
 
    [Test]
    public void ShouldOffsetRectangleAsProperty() {
      TestRect propertyRect = new TestRect();
      propertyRect.Rectangle = new RectangleF(0, 0, 10, 10);
      propertyRect.Rectangle.Offset(1, 1);
      Assert.AreEqual(1, propertyRect.Rectangle.X, "propertyRect failed");
    }
 
    private class TestRect {
      public RectangleF Rectangle { get; set; }
    }

The second test fails... it says that the propertyRect.Rectangle.X is still 0.

Any ideas why this behavior might be? I am stumped at this point.

Obviously I can get around it by offsetting the Rectangle before assigning it, I am just trying to figure out why I am seeing this behavior.

Thanks,

Matt