Our last post, “Count vs Any To Check A If A Collection Has Elements In C# & .NET”, looked at considerations around determining the length or size of a collection.

In this post, we will look at a potential middle ground that will allow you to check whether the computations of a collection length are expensive beforehand.

This can be achieved using the LINQ TryGetNonEnumeratedCount extension method.

This method does various checks to determine if the enumeration of the collection can be done cheaply. If it can, it returns true as well as the count. Otherwise, it returns false.

A false return means that the enumeration is a potentially expensive operation, and if you really want to do it, you can proceed at your own risk.

Take, for example, an array.

string[] people = ["Peter", "Bartholomew", "Matthew", "Simon", "Jude"];

if (people.TryGetNonEnumeratedCount(out var count))
{
	Console.WriteLine($"This is a cheap operation - we got {count}");
}

If we run this, the following will be printed:

This is a cheap operation - we got 5

Another example is this expression:

var complex = Enumerable.Range(1, 50).Where(n => n % 2 == 0);

if (complex.TryGetNonEnumeratedCount(out var _))
{
	Console.WriteLine($"This is a cheap operation");
}
else
{
	Console.WriteLine("This is expensive!");
}

To count the number of elements is a potentially expensive operation, as the modulus division must be done for all the elements.

Running this code will print the following:

This is expensive!

You can therefore decide whether or not to perform the enumeration anyway in the else block.

var complex = Enumerable.Range(1, 50).Where(n => n % 2 == 0);

if (complex.TryGetNonEnumeratedCount(out var _))
{
	Console.WriteLine($"This is a cheap operation");
}
else
{
  Console.WriteLine("This is expensive!");
  //
  // Have our enumeration here
  //
  var result = complex.ToList();

  Console.WriteLine($"There are {result.Count} even numbers");
}

TLDR

The LINQ TryGetNonEnumeratedCount extension method allows you to check whether or not enumerating a collection to determine its length is an expensive operation.

The code is in my GitHub.

Happy hacking!