Why structure behaves differently in vb and c#?

Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
        Dim c As System.Drawing.Color
        If c <> Nothing Then
            MessageBox.Show(c.Name)
        Else
            MessageBox.Show("null")
        End If
    End Sub
    Public Enum Align
        Left
        Center
        Right
    End Enum
    Public Alignment As Align
    Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click
        If Alignment <> Nothing Then
            MessageBox.Show("true")
        Else
            MessageBox.Show("false")
        End If
    End Sub

private void button1_Click(object sender, EventArgs e)
        {
            System.Drawing.Color c = default(System.Drawing.Color);
            if (c != null)
            {
                MessageBox.Show(c.Name);
            }
            else
            {
                MessageBox.Show("null");
            }
        }
        public enum Align
        {
            Left,
            Center,
            Right
        }

        public Align Alignment;

        private void button2_Click(object sender, EventArgs e)
        {
            if (Alignment != null)
            {
                MessageBox.Show("true");
            }
            else
            {
                MessageBox.Show("false");
            }
        }