[DefaultValue((byte)0)]
public byte Transparency { get; set; }

<DefaultValue(CType(0, Byte))>
Public Property Transparency As Byte

private void Form1_Load(object sender, EventArgs e) {
    ImageLayer layer = new ImageLayer();
    BingMapDataProvider provider = new BingMapDataProvider { BingKey = "Your key here." };
    layer.DataProvider = provider;
    layer.Transparency = 120;
    mapControl1.Layers.Add(layer);
}

Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs)
    Dim layer As ImageLayer = New ImageLayer()
    Dim provider As BingMapDataProvider = New BingMapDataProvider With {
        .BingKey = "Your key here."
    }
    layer.DataProvider = provider
    layer.Transparency = 120
    mapControl1.Layers.Add(layer)
End Sub