Lambda expression to return zero if null

Question

in my WPF application i used to add controls dynamically to a Canvas. The name format of the control is "Control_UniqueValue".

i.e., if i add first control to the canvas, then the name will be "Control_1" and the next will be "Control_2" etc...

my requirement is to get the max value of the added controls

i used the following statement for that

string maxId = (string)canvas1.Children.Cast<FrameworkElement>().ToList().Max(x => (x.Name.Substring(x.Name.LastIndexOf('_') + 1)));

but the problem here is

need to return the value as int
if the canvas contains no controls it will raise error (tried using Nullable type, but failed)

Lee · Accepted Answer · 2010-12-28 12:06:15Z

4

int  maxId = canvas1.Children
    .Cast<FrameworkElement>()
    .Select(e => int.Parse(e.Name.Substring(e.Name.LastIndexOf('_'))))
    .DefaultIfEmpty()
    .Max();

This should return 0 instead of throwing an exception if there are no elements in the sequence. Also, the call to ToList in your code is not required. This will still throw an exception if any of the control names are not in the expected format.

edited Dec 28, 2010 at 12:06

answered Dec 28, 2010 at 11:42

Lee

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Jan · Accepted Answer · 2010-12-28 12:06:52Z

4

To get an integer, you might use the int.Parse method. You don't have to call ToList().

There is no error checking, so your controls have to named regarding to your rule name_id.

You can use the DefaultIfEmpty extension method to provide a default value, if the sequence is empty:

int maxId = canvas1.Children
             .Cast<FrameworkElement>()
             .DefaultIfEmpty(new FrameworkElement() { Name = "Control_0" })
             .Max(x => (int.Parse(x.Name.Split('_')[1]) + 1)));

edited Dec 28, 2010 at 12:06

answered Dec 28, 2010 at 11:38

Jan

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2 Comments

Binil Over a year ago

The type arguments for method 'System.Linq.Enumerable.DefaultIfEmpty<TSource>(System.Collections.Generic.IEnumerable<TSource>, TSource)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

Jan Over a year ago

I have edited my answer. You have to provide an instance of the FrameworkElement class as default value for the empty list of cause.

Isak Savo · Accepted Answer · 2010-12-28 11:42:01Z

Since Max() requires at least one element in its sequence you either have to

catch the exception and set max to zero
check the length before calling Max(). I'd recommend this

While it is possible to write the logic in one long query expression, I'd recommend you break it up a bit to improve readability. Create a method that returns the integer part of a child control's name:

private int NumberFromElementName(string name)
{
    // Or search for the last '_' using name.LastIndexOf()
    var numString = name.Substring("Control_".Length);
    return Int32.Parse(numString);
}

Then do the query in two steps to allow you to look at the length of the returned sequence. Notice I convert it to an array to avoid having to run the query twice. I'm also using the OfType extension method so that it works even if the canvas has a child which isn't of type FrameworkElement.

var children = canvas1.Children.OfType<FrameworkElement>().ToArray();
int max = 0;
if (children.Length > 0)
    max = children.Max(x => NumberFromElementName(x.Name));
string nextChildName = String.Format ("Control_{0}", max + 1);

EDIT: As pointed out by @Jan in his answer, you can avoid splitting the query in two parts by using the DefaultIfEmpty before the Max call in the query.

Binil · Accepted Answer · 2010-12-28 12:26:47Z

0

got the final answer

int maxId = canvas1.Children .Cast<FrameworkElement>() .Select(e => int.Parse(e.Name.Substring(e.Name.LastIndexOf('_')+ 1))) .DefaultIfEmpty() .Max(x => x + 1);

Thanks Jan & Lee

answered Dec 28, 2010 at 12:26

Binil

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