Expression in LINQ
Take the reference of System.Linq.Expressions namespace and use an Expression<TDelegate> class to define an Expression. Expression<TDelegate> requires delegate type Func or Action.
For example, you can assign lambda expression to the isTeenAger variable of Func type delegate, as shown below:
Define Func delegate for an expression in C#
public class Student
{
public int StudentID { get; set; }
public string StudentName { get; set; }
public int Age { get; set; }
}
Func<Student, bool> isTeenAger = s => s.Age > 12 && s.Age < 20;
And now, you can convert the above Func type delegate into an Expression by wrapping Func delegate with Expression, as below:
Define Expression in C#
Expression<Func<Student, bool>> isTeenAgerExpr = s => s.Age > 12 && s.Age < 20;
In the same way, you can also wrap an Action<t> type delegate with Expression if you don't return a value from the delegate.
Define Expression in C#
Expression<Action<Student>> printStudentName = s => Console.WriteLine(s.StudentName);
You can invoke the delegate wrapped by an Expression the same way as a delegate, but first you need to compile it using the Compile() method. Compile() returns delegateof Func or Action type so that you can invoke it like a delegate.
Invoke Expression in C#
Expression<Func<Student, bool>> isTeenAgerExpr = s => s.Age > 12 && s.Age < 20;
//compile Expression using Compile method to invoke it as Delegate
Func<Student, bool> isTeenAger = isTeenAgerExpr.Compile();
//Invoke
bool result = isTeenAger(new Student(){ StudentID = 1, StudentName = "Steve", Age = 20});
Output: false