Option Strict On 禁止 String 和 Object {String} 之间的隐式转换

Question

I am trying to clean my ways as a beginner programmer by using Option Strict On . I managed to clean all errors except this one.
I am using the Tag property of a ToolStrip for some text information. Clicking on the ToolStrip, I need to remember the value of the Tag in a string and change the value of that Tag.

How can I convert the Object {String} sender.tag to a String and the String myString and an Object {String} ?

Private Sub ToolStrip_ItemClicked(sender As Object, e As ToolStripItemClickedEventArgs) Handles ToolStrip.ItemClicked
    Dim myString As String = sender.tag
    sender.tag = "It is selected"
    'more code...
End Sub

Edit: see here a screenshot of the relevant part of the code:

Answer 1

If you want to read the Tag Property of a ToolstripItem , you need to read the Tag of the Item selected. This is provided by the e.ClickedItem property of the ToolStripItemClickedEventArgs object:

Gets the item that was clicked on the ToolStrip.

If you want to read the Tag property of the ToolStrip this Item belongs to, cast sender to ToolStrip (or Control, the base class this property belongs to) and cast its Tag property to String.

Note: when you double-click a ToolStrip Control, an ItemClicked handler is created because this is the default event of that Control . The event is related to the ToolStrip object, so the sender object in the event handler will be the reference of the ToolStrip instance that raised the event.

The Tag Property is of Type Object , that's why you need to cast that, too.

Private Sub ToolStrip_ItemClicked(sender As Object, e As ToolStripItemClickedEventArgs) Handles ToolStrip.ItemClicked
    Dim itemTagAsString As String = e.ClickedItem.Tag?.ToString()

    Dim toolStripTagAsString = DirectCast(sender, ToolStrip).Tag?.ToString()
    ' Or, cast to Control, the Tag Property belongs to the base class
    Dim toolStripTagAsString = DirectCast(sender, Control).Tag?.ToString()
End Sub

Check the Type of itemTtag in these examples (with Option Infer On ):

Dim itemTtag = e.ClickedItem.Tag
Dim itemTtag = e.ClickedItem.Tag?.ToString()

So it's more clear why this is not a valid statement:

Dim itemTtag As String = e.ClickedItem.Tag

Note: the null conditional operator ( ? , aka Elvis), is used here because the Tag could be null ( Nothing ). If it is, you won't get an exception, just an empty string.

Answer 2

Good that you have titled your question generically. And you need a generic answer.

Option Strict On is a good thing. It makes harder to code but performance at runtime will increase because there will be less implicit data type conversions.

Lets take your code ...sender As Object... and ...sender.tag.. This is a typical thing in .net. Often you see a parameter of type object , which means, any data type can be passed. Object does not have all the properties and methods defined that belong to that data type.

For example

Dim oTxt as object = new TextBox()

oTxt will not automatically have property Text . You need to cast . When you know 100% the object type, do

dim str as string = DirectCast(oTxt, TextBox).Text

But sometimes you don't know what Type is in your object . In this case you try casting and then check for null

dim txtBx as TextBox = TryCast(oTxt, TextBox)
if txtBx IsNot Nothing Then str = txtBx.Text

Your real problem is that you need to understand type casting. You should cast it explicitly even if you don't have Options Strinct On because when you do x = sender.tag implicitly, you really using a late binding , which is bad for performance. And this also opens doors for potential runtime errors.

Your topics of research should be: type casting, late binding, boxing/unboxing