What is TKey in System.Linq.Expressions.Expression<Func<TSource,TKey>>?

Question

I'm trying to store expressions required by Linq OrderBy clauses in data structures, so I can just go query = query.OrderBy(MySortExpression);

OrderBy takes System.Linq.Expressions.Expression<Func<TSource,TKey>> as a parameter. TSource is the entity type you are sorting on, but what type is TKey supposed to be?

Answer 1

TKey is a generic type argument denoting the type of the expression on which you sort. For example, if you sort strings by length, TSource will be string , and TKey will be int , as in the code below:

string [] myStrings = new[] {"quick", "brown", "fox", "jumps"};
var ordered = myStrings.OrderBy(s => s.Length);

Answer 2

TKey is the type of the return type of the expression. For instance:

users.OrderBy(user => user.Name); 

As Name is string, the type will be System.Linq.Expressions.Expression<Func<User,string>>

Answer 3

The TKey is not bound to a specific type. Usually it is a projection of a property to a primitive type to enable sorting.

Assuming a Person with a BirthYear property you would select

 persons.OrderBy(p => p.BirthYear);