MapControl.Layers Property

public LayerCollection Layers { get; }

Public ReadOnly Property Layers As LayerCollection

mapControl1.Layers.Add(new ImageLayer() {
    DataProvider = new BingMapDataProvider() {
        BingKey = "YOUR BING KEY"
    }
});

mapControl1.Layers.Add(New ImageLayer() With { _
    .DataProvider = New BingMapDataProvider() With {.BingKey = "YOUR BING KEY"} _
})