So, I was considering a problem I had that called for a LINQ statement that contains a FirstOrDefault() call. If there was an object returned I wanted a string property from it. If not, then I wanted to use a string.Empty.
I’ve often just caputured the result of the LINQ statement, checked for null, and if I had an object I’d call the property.
e.g.
var number = numbers.FirstOrDefault();
string firstNumberCaption = string.Empty;
if (number != null)
firstNumberCaption = number.Caption;
Console.WriteLine("The first number is {0}", firstNumberCaption);
However, that code is a bit convoluted, and it occurred to me that there would be a better way of doing this.
The pure LINQ way
What if I called Select before the FirstOrDefault() to get the value of the property that I wanted. That way I don’t have to worry about implementing my check.
var firstNumber = numbers
.Select(n => n.Caption)
.FirstOrDefault();
And if I am desperate for the resulting string to be the empty string I can always append ?? string.Empty onto the end, like this:
var firstNumber = numbers
.Select(n => n.Caption)
.FirstOrDefault() ?? string.Empty;
If you are not familiar with the ?? (null-coalescing) operator you can find out more on MSDN.
Optimisation
My next concern was that perhaps it will convert every instance in the enumerable that was passed to LINQ before discarding them all but the first one. However, that doesn’t happen. For example, the following code only outputs one thing:
var firstNumber = numbers
.Select(n =>
{
Console.WriteLine("Select {0} / {1}", n.Value, n.Caption);
return n.Caption;
})
.FirstOrDefault() ?? string.Empty;
Filtering
I often put filters in my calls to FirstOrDefault(), and most of the time it is a filter on something other than what I’m returning in the Select part of the statement. Obviously, if we are performing the Select first, then the filter is not going to work if it is looking for data that is no longer there. In this case we just insert a Where statement just before the select to ensure that the filtering does happen.
So, for example, the first number in the sequence greater than 2:
var firstNumberCaption = numbers
.Where(n => n.Value > 2)
.Select(n => n.Caption)
.FirstOrDefault() ?? string.Empty;
Console.WriteLine("The first number is {0}", firstNumberCaption);
Full program
Here is the full program (using the last example) if you want to see everything that is going on.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ConsoleApplication2
{
class Number
{
public Number(int value, string caption)
{
Value = value;
Caption = caption;
}
public int Value { get; set; }
public string Caption { get; set; }
}
class Program
{
static void Main(string[] args)
{
Number[] numbers =
{
new Number(1, "One"),
new Number(2, "Two"),
new Number(3, "Three")
};
var firstNumberCaption = numbers
.Where(n => n.Value > 2)
.Select(n => n.Caption)
.FirstOrDefault() ?? string.Empty;
Console.WriteLine("The first number is {0}", firstNumberCaption);
Console.ReadLine();
}
}
}
The Blog of Colin Mackay 