My question is why the designer does not store DesignTime values in case of using Structure instead of Class.

Here is my code

<TypeConverter(GetType(ExpandableObjectConverter))>
Public Structure ABC
    Public Property Prop1 As Integer
    Public Property Prop2 As String
    Public Property Prop3 As Boolean
    Public Overrides Function ToString() As String
        Return Prop2
    End Function
End Structure
<TypeConverter(GetType(ExpandableObjectConverter))>
Public Class XYZ
    Public Property Prop1 As Integer
    Public Property Prop2 As String
    Public Property Prop3 As Boolean
    Public Overrides Function ToString() As String
        Return Prop2
    End Function
End Class
Public Class MyControl
    Inherits Control

    <DesignerSerializationVisibility(DesignerSerializationVisibility.Content)>
    Public ReadOnly Property ABCs As New ObjectModel.Collection(Of ABC)

    <DesignerSerializationVisibility(DesignerSerializationVisibility.Content)>
    Public ReadOnly Property XYZs As New ObjectModel.Collection(Of XYZ)

    Protected Overrides Sub OnClick(e As EventArgs)
        For Each a As ABC In ABCs
            Debug.WriteLine("ABC :" & a.Prop2)
        Next

        For Each x As XYZ In XYZs
            Debug.WriteLine("XYZ :" & x.Prop2)
        Next
    End Sub
End Class

The designer only stores XYZs values and can not store ABCs

Here is the designer code

'MyControl1
'
Me.MyControl1.BackColor = System.Drawing.Color.WhiteSmoke
Me.MyControl1.ForeColor = System.Drawing.Color.Gray
Me.MyControl1.Location = New System.Drawing.Point(150, 216)
Me.MyControl1.Name = "MyControl1"
Me.MyControl1.Size = New System.Drawing.Size(182, 123)
Me.MyControl1.TabIndex = 2
Me.MyControl1.Text = "MyControl1"
Xyz1.Prop1 = 0
Xyz1.Prop2 = "Test"
Xyz1.Prop3 = False
Me.MyControl1.XYZs.Add(Xyz1)