How to dynamically add different Commands to buttons

Question

I want to dynamically add Commands to buttons. Each button should execute a function using the iterator variable as a parameter. The problem is, while the button text displays correctly, "0, 1, 2" on the buttons, the commands are all executed with the final value of the iterator, "SomeCommand(2)". How do I get the buttons to execute SomeCommand(0), SomeCommand(1) and SomeCommand(2) respectively?

public void DynamicButtons()
{
  for(int i = 0; i < 3; i++)
  {
    Button newButton = new Button { Command = new Command(() => { SomeFunction(i); }), Text = i.ToString() };
  }
}

Patrick Allwood · Accepted Answer · 2018-10-16 09:14:38Z

3

You need to create a copy of your loop index and pass that.

When you create a lambda it stores a reference to the thing in the outer scope, not the value itself. So as your loop increments, everything has a reference to the counter, and so everything ends up with the value '2' when it comes to execute the command.

public void DynamicButtons()
{
  for(int i = 0; i < 3; i++)
  {
    var copy = i;

    Button newButton = new Button 
    { 
        Command = new Command(() => 
        { 
            SomeFunction(copy); 
        }), 
        Text = i.ToString() 
    };
  }
}

Further reading: https://blogs.msdn.microsoft.com/ericlippert/tag/closures/

answered Oct 16, 2018 at 9:14

Patrick Allwood

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