C#, deleting all the controls with the same .Tag

Question

My problem is that when I compare two, absolutely the same tags, they turn out to be different instead.

The description of a code: I create 3 controls (button, label and a textbox) and assign the same tag (let's say, 0) to them all. When I press this newly created button, I want to delete all of the 3 controls with the same tag.

Code for adding the buttons (simplified):

int Count = 0; // This var changes, but for the example it is 0

Button newButton = new Button();
newButton.Tag = Count;
newButton.Click += new EventHandler(DeleteName);
Controls.Add(newButton);

Label newLabel = new Label();
newLabel.Tag = Count;
Controls.Add(newLabel);

And the same for a TextBox.

The coding behind deleting:

private void DeleteName(object sender, EventArgs e)
        {
            List<Control> toDelete = new List<Control>();

            Button btn = sender as Button;

            foreach (Control c in Controls) 
            {
                if (c.Tag == btn.Tag)
                toDelete.Add(c);
            }

            int tmp = toDelete.Count;

            for (int i = 0; i < tmp; i++)
            {
                Controls.Remove(toDelete[i]);
            }
        }

It used to work perfectly when I did the same logic before, but now it just deletes the button and no other control (the textbox and label stay untouched). When I replace "if (c.Tag == btn.Tag)" with, for example, "if (c is TextBox)", it adds all the TextBoxes to the list and deletes them, so I believe the problem is in this comparison of Tags.

Answer 1

The likely reason you're running into this is that Tag is an object, so the equality comparison will do a reference comparison, but you're setting it to a value type. And even if you were setting it to a reference type, the comparison would only return true if they were pointing to the exact same object.

One way to solve this would be to cast the Tag to the appropriate type, or, more commonly, use the ToString() method for comparison. We should also add null conditional operators in the check, in case a Tag is null, to avoid a NullReferenceException:

if (c.Tag?.ToString() == btn.Tag?.ToString())

---

Another thing you might consider doing is creating a separate method to delete all controls with a particular tag, and another one that recursively finds all controls with that tag. This will help out if you are in a situation where you have some controls with the "delete" tag that are children of a different container.

For example:

// Returns all controls under the parent with the specified tag
private List<Control> GetTaggedControls(string tag, Control parent)
{
    var taggedControls = new List<Control>();

    foreach (Control control in parent.Controls)
    {
        if (control.Tag?.ToString() == tag)
        {                
            taggedControls.Add(control);
        }

        // Recursively call this method in case this is a container
        taggedControls.AddRange(GetTaggedControls(tag, control));
    }

    return taggedControls;
}

// Deletes all controls with the specified tag
private void DeleteControlsWithTag(string tag)
{
    foreach (Control control in GetTaggedControls(tag, this))
    {
        Controls.Remove(control);
    }
}

For example, if you add the code below to a new WinForm project, it will create 10 labels with every other one containing a Tag with "deleteMe". It also creates a button that, when clicked, calls the DeleteControlsWithTag method above:

private void Form1_Load(object sender, EventArgs e)
{
    var padSpace = 5;
    var height = 15;

    // Add 10 labels, with a common tag on every other one
    for (int i = 0; i < 10; i++)
    {
        Controls.Add(new Label
        {
            Text = $"Label {i}",
            Top = padSpace * (i + 1) + height * i,
            Height = height,
            Visible = true,
            Tag = i % 2 == 0 ? "deleteMe" : "saveMe"
        });
    }

    // Add a button that will delete controls with deleteMe tag
    var btn = new Button
    {
        Text = "Delete Even Labels",
        Height = 20,
        Width = 150,
        Top = padSpace * 11 + height * 10,
        Visible = true
    };

    btn.Click += (o, ea) => { DeleteControlsWithTag("deleteMe"); };

    Controls.Add(btn);
}