Windows Forms Element Host Fails To Display WPF User Control

Today I had the situation whereby my Windows Form would not display a WPF User Control within its Element Host Control. The source of the problem was that my WinForms project was set to target the .NET Framework 4 Client Profile. The solution was to change the setting under the compile options for the project to .NET Framework 4. Everything now works fine, although you may need to delete forms and user controls and recreate them against the new target framework.