Lambda Expressions

• Creating a method for each and every BindValue can become cumbersome

• Imagine if we had 10 or 20 of these.

• Also - to see the actual implementation of the method, we would need to go back and forth between our constructor and the GetDaysBetween and GetEndOfMonthOfFromDate methods themselves

• Also every time we create such a method we have to think of a name for it, and than only use it once

• That's why in C# they came up with a term called anonymous methods also known as lambda expressions

• anonymous methods aka lambda expressions are methods that are written in line, where they are used

• Simply take the C#6 style method without it's name and put it in place.

• In our code we have the GetDaysBetween and GetEndOfMonthOfFromDate method that are only used once.

public class DemoLocalColumns : UIControllerBase
{
    public readonly DateColumn FromDate = new DateColumn("From Date");
    public readonly DateColumn ToDate = new DateColumn("To Date");
    public readonly NumberColumn DaysBetween = new NumberColumn("Days Between","5CN");

    public DemoLocalColumns()
    {
-       ToDate.BindValue(GetEndOfMonthOfFromDate);
        ToDate.BindValue(() => FromDate.EndOfMonth);
-       DaysBetween.BindValue(GetDaysBetween);
        DaysBetween.BindValue(() => ToDate - FromDate);
    }
-   public Date GetEndOfMonthOfFromDate() => FromDate.EndOfMonth;
-   public Number GetDaysBetween() => ToDate - FromDate;
    ...
} 

• Once you get used to it, it's easier to read and easier to write (a lot less code)

Multiline vs single line

• A Single line lambda expression is written like this:

public class DemoLocalColumns : UIControllerBase
{
    public readonly DateColumn FromDate = new DateColumn("From Date");
    public readonly DateColumn ToDate = new DateColumn("To Date");
    public readonly NumberColumn DaysBetween = new NumberColumn("Days Between","5CN");

    public DemoLocalColumns()
    {
        ToDate.BindValue(() => FromDate.EndOfMonth);
        DaysBetween.BindValue(() => ToDate - FromDate);
    }
  ...
} 

• A multi line lambda expression is written like this:

public class DemoLocalColumns : UIControllerBase
{
    public readonly DateColumn FromDate = new DateColumn("From Date");
    public readonly DateColumn ToDate = new DateColumn("To Date");
    public readonly NumberColumn DaysBetween = new NumberColumn("Days Between","5CN");

    public DemoLocalColumns()
    {
        ToDate.BindValue(() => FromDate.EndOfMonth);
        DaysBetween.BindValue(() => 
        {
            Debug.WriteLine("Evaluating To Date");
            return ToDate - FromDate;
        });
    }
  ...
} 

• A single line lambda expression doesn't need the curly brackets, and it doesn't need the keyword return to return a value
• A multi line lambda expression needs the curly brackets, and it needs the return keyword to return a value

For a deeper discussion of these topics see Lambda Expressions Generics and BindValue

• Exercise Updating Data Programmatically

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