Embedded local functions in C# 7.0

Occasionally it happens that we want to group related code within a function without creating another private function for that. C# 7.0 provides a neat way for that in the form of local functions which are functions within functions.

Consider the following example:

public class LocalFunctionsDemo
{
	public void RunDemo()
	{
		int GetDiff(DateTime first, DateTime second)
		{
			TimeSpan ts = (second - first).Duration();
			return Convert.ToInt32(ts.TotalDays);
		}

                int dayDiff = GetDiff(DateTime.UtcNow, DateTime.UtcNow.AddHours(10));
			
		Console.WriteLine(dayDiff);
	}
}

Note how GetDiff lies within the boundaries of the RunDemo method. It can only be called from within the declaring function and not from anywhere else, not even from within the containing class. In this sense GetDiff is sort of a super-private function with a very limited scope.

Curiously we can call local functions BEFORE they are declared. The following code is still valid:

public void RunDemo()
{
	int dayDiff = GetDiff(DateTime.UtcNow, DateTime.UtcNow.AddHours(10));

	int GetDiff(DateTime first, DateTime second)
	{
		TimeSpan ts = (second - first).Duration();
		return Convert.ToInt32(ts.TotalDays);
	}
			
	Console.WriteLine(dayDiff);
}

View all various C# language feature related posts here.