.NET Framework 4.6 and 4.5
protected override Size MeasureOverride(
The parent UIElement for Viewbox typically calls this method, which performs the first layout pass.
Viewbox measures its child at an infinite constraint, allowing the child to be as large as it requires.
Supported in: 4.6, 4.5, 4, 3.5, 3.0
.NET Framework Client Profile
Supported in: 4, 3.5 SP1