how to generate qr code in asp.net using c# Event Handling in Visual C#.NET

Print QR-Code in Visual C#.NET Event Handling

Event Handling
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To respond to user input in Windows Forms applications, we can just use ordinary C# event handling user interface elements are all represented as objects, and they raise events in the usual way whenever anything interesting happens. As you already saw with the binding source change notifications, Visual Studio can generate the eventhandling code for us.
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Controls define a default event, usually the event you are most likely to want to handle for the control in question. So we can handle the Click events of the two buttons by double-clicking on them in the designer. Visual Studio will add event handler methods with suitable names and signatures, and will add the code to handle the event in the generated part of the partial class. We simply need to provide the code. The New button handler is pretty straightforward, because we already wrote the code to add a new item:
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private void newButton_Click(object sender, EventArgs e) { CreateNewItem(); }
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Deletion is slightly more involved:
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private void deleteButton_Click(object sender, EventArgs e) { if (entriesListView.SelectedIndices.Count != 0) { int entryIndex = entriesListView.SelectedIndices[0]; entriesSource.RemoveAt(entryIndex); } }
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The ListView control is able to support multiple selection. We ve disabled this, but we still have to negotiate an API designed to support it it offers a SelectedIndices collection, providing all the selected items. We just make sure it s not empty, and then use the first index. We remove the object via the binding source so that the data binding system knows what s going on, just as we did when adding a new item. (In fact, it would work if we didn t do this because this example uses a BindingList to hold the model, and that raises change notifications. Unlike AddNew, there s no particular advantage to going via the binding source here, but since we re treating the binding source as the central point through which all changes are handled, it s good to be consistent.) We can now add multiple entries. This reveals a missing piece we have done nothing yet to ensure that when the user selects an item in the list view, the corresponding entry s properties appear in the rest of the form. So we need to add a handler to the list view s SelectedIndexChanged event. That s its default event, so you can just doubleclick the list view in the form designer. Then all we need to do is set the binding source s Position property:
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private void entriesListView_SelectedIndexChanged(object sender, EventArgs e) { if (entriesListView.SelectedIndices.Count != 0) { int entryIndex = entriesListView.SelectedIndices[0]; entriesSource.Position = entryIndex; } }
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We ve had to jump through the same hoop to get the selected item index. There s really just one line of interest here the one that sets the Position.
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We have to do this only because the ListView doesn t do data binding automatically. The ListBox and most data grid controls will automatically synchronize the current data binding position with the selected item.
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One little bug remains. When we delete all the items, the text boxes and date picker are bound to nothing. This doesn t crash the program; it just means the user can type in details that go nowhere. There are a couple of ways we could fix this. In the list change notification handler, we could look at the number of entries, and disable everything except the New button to make it clear that there s nothing to edit right now. Or we could handle change notifications in the text boxes text box controls raise a TextChanged event, and we could handle that (as well as changes to the date picker or description) and create a new entry if the user types into an empty list. Since neither of these would illustrate anything you haven t already seen we ll leave this part up to you.
Summary
Windows Forms provides a .NET wrapper around the Win32 user interface model. Visible elements are represented as objects that derive from the Control base class. Control features are configured with normal .NET properties, and we can use ordinary C# event handlers to respond to user input. Data binding automates some aspects of connecting data to the screen, although the support in Windows Forms is not as comprehensive as in WPF, and we needed to do some extra work to get the effect we required on the control we wanted to use. Windows Forms may be less powerful than WPF, but it has a smaller memory footprint and may be more suitable on low-end hardware, or if you need to use controls that are available only for Windows Forms.
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