Sometimes functions contain repetitive code that lends itself to being broken up into smaller functions however you were limited to either putting them into a private method that no other function should call or creating a lambda to re-use through the function.

C# 7.0 allows functions to contain other functions to aid in readability and re-use without external methods having access to them. They additionally have access to all the variables in scope of the parent function.

Code

Now

public int GetYearsApart()
{
    var now = DateTime.Now;
    var age1 = CalculateAge(first.DateOfBirth);
    var age2 = CalculateAge(second.DateOfBirth);
    return Math.Abs(age1 - age2);

    int CalculateAge(DateTime dob)
    {
        return now.Year - dob.Year -
            (now.Month < dob.Month || (now.Month == dob.Month && now.Day < dob.Day) ? 1 : 0);
    }
}

Before (private method)

public int GetYearsApart()
{
    var now = DateTime.Now;
    var age1 = CalculateAge(first.DateOfBirth, now);
    var age2 = CalculateAge(second.DateOfBirth, now);
    return Math.Abs(age1 - age2);
}

private int CalculateAge(DateTime dob, DateTime now)
{
    return now.Year - dob.Year -
        (now.Month < dob.Month || (now.Month == dob.Month && now.Day < dob.Day) ? 1 : 0);
}

Before (local lambda)

public int GetYearsApart()
{
    var now = DateTime.Now;
    Func<DateTime, int> calculateAge = dob =>
        now.Year - dob.Year -
        (now.Month < dob.Month || (now.Month == dob.Month && now.Day < dob.Day) ? 1 : 0);

    var age1 = calculateAge(first.DateOfBirth);
    var age2 = calculateAge(second.DateOfBirth);
    return Math.Abs(age1 - age2);
}

Notes

• Unlike a lambda, a local function can be called before it is declared, can be recursive or generic, can be an iterator (yield return) and avoids allocating a delegate
• Static local functions ^{C# 8.0} extends this functionality to allow them to be scoped static where they will not automatically capture the parent function's scope
• Attributes on local functions ^{C# 9.0} allows attributes on local functions and their parameters

More information

• What's new in C# 7.0
• Local functions vs. lambda expressions