Guide to the Language

 Delegate Signature
As mentioned, a method can be assigned to a delegate object if it matches 
the delegate’s signature. However, a method does not have to match 
the signature exactly. A delegate object can also refer to a method that 
has a more derived return type than that defined in the delegate, or that 
has parameter types that are ancestors of the corresponding delegate’s 
parameter types.
class Base {}
class Derived : Base {}
delegate Base MyDelegate(Derived d);
class MyClass
{
static Derived Test(Base o)
{
return new Derived();
}
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static void Main()
{
MyDelegate d = Test;
}
}
 Delegates as Parameters
An important property of delegates is that they can be passed as method 
parameters. To demonstrate the benefit of this, two simple classes will 
be defined. The first one is a data storage class called PersonDB that has 
an array containing a couple of names. It also has a method that takes a 
delegate object as its argument and calls that delegate for each name in the 
array.
delegate void ProcessPersonDelegate(string name);
class PersonDB
{
string[] list = { "John", "Sam", "Dave" };
public void Process(ProcessPersonDelegate f)
{
foreach(string s in list) f(s);
}
}
The second class is Client, which will use the storage class. It has a 
Main method that creates an instance of PersonDB, and it calls that object’s 
Process method with a method that is defined in the Client class.
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class Client
{
static void Main()
{
PersonDB p = new PersonDB();
p.Process(PrintName);
}
static void PrintName(string name)
{
System.Console.WriteLine(name);
}
}
The benefit of this approach is that it allows the implementation 
of the data storage to be separated from the implementation of the 
data processing. The storage class only handles the storage and has no 
knowledge of the processing that is done on the data. This allows the 
storage class to be written in a more general way than if this class had to 
implement all of the potential processing operations that a client may want 
to perform on the data. With this solution, the client can simply plug its 
own processing code into the existing storage class.
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© Mikael Olsson 2020 
M. Olsson, C# 8 Quick Syntax Reference, 
https://doi.org/10.1007/978-1-4842-5577-3_27