If you use method syntax, you'll see func quite often, for instance in Where, GroupBy, Join, etc
Every method with some input parameters and one return value can be translated to a Func<...> as follows
MyReturnType DoSomething(ParameterType1 p1, ParameterType2, p2) {...}
Func<ParameterType1, ParameterType2, MyReturnType> myFunc = (x, y) => DoSomething(x, y);
The part Func<ParameterType1, ParameterType2, MyReturnType> means: a function with two input parameters and one return value. The input parameters are of type ParameterType1 and ParameterType2, in this order. The return value is of MyReturnType.
You instantiate an object of Func<ParameterType1, ParameterType2, MyReturnType> using a lambda expression. Before the => you type a declaration for the input parameters, after the => you call the function with these input parameters. If you have more than one input parameter you make them comma separated surrounded by brackets.
For a Where you need a Func<TSource, bool>. So a function that has as input one source element, and as result a bool:
Where(x => x.Name == "John Doe")
For a GroupJoin you need a resultSelector of type Func<TOuter,System.Collections.Generic.IEnumerable<TInner>,TResult> resultSelector
So this is a function with as input one element of the outer sequence, and a sequence of elements of the inner sequence. For example, to query Teachers with their Students:
var result = Teachers.GroupJoin(Students,
teacher => teacher.Id, // from every Teacher take the Id,
student => student.TeacherId, // from every Student take the TeacherId,
(teacher, students) => new
{
Id = teacher.Id,
Name = teacher.Name,
Students = students.Select(student => new
{
Id = student.Id,
Name = student.Name,
})
.ToList(),
});
Here you see several Funcs. TOuter is Teacher, TInner is Student, TKey is int
- OuterKeySelector:
Func<TOuter, TKey>: teacher => teacher.Id
- InnerKeySelector:
Func<TInner, TKey>: student => student.TeacherId
- ResultSelector:
Func<Touter, IEnumerable<TInner>, TResult>
The resultSelector is a function that takes one TOuter (a Teacher), and a sequence of TInner (all students of this Teacher) and creates one object using the input parameters
(teacher, students) => new {... use teacher and students }
When creating the lambda expression it is often helpful if you use plurals to refer to collections (teachers, students) and singulars if you refer one element of a collection (student).
Use the => to start defining the func. You can use input parameters that were defined before the => to define the result after the =>
