C# - the method takes another method as a parameter, how to understand?

Question

ASP.NET

I'm studying unit tests and found a gap in my basic C# knowledge.
Here is the method
mock.Setup( m => m.ApplyDiscount(It.IsAny()) )
.Returns(total => total);
In this case, Setup takes a lambda expression m => m.ApplyDiscount(It.IsAny()) can anyone explain how to understand this? Give the name of the topic that I need to learn in order to understand such expressions.
Do I guess correctly that Setup is not a method but a delegate?

Answer 1

Setupis the usual method.
Do Returnsnot pay attention, it is also a regular method, it is by itself, it's just Fluid, delegates and lambdas have nothing to do with it.
But what is written in the form of a lambda expression is a delegate.
I wrote an example here with four similar syntaxes that differ in appearance, but are essentially equivalent, if you reproduce it (create a WinForms application, a form and a button with a handler), you will understand what it is and approximately why it can be used:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {

        public delegate void EventHandler2(
            int i
        );

        public Form1()
        {
            InitializeComponent();
        }

        void setEventHandler(EventHandler2 eh)
        {
            eh.Invoke(123);
        }

        private void button1_Click(object sender, EventArgs e)
        {
            //Example 1
            setEventHandler(x => MessageBox.Show(x.ToString()));

            //Example 2 (equalent)
            setEventHandler((x) => { MessageBox.Show(x.ToString()); });

            //Example 3 (equalent)
            setEventHandler(delegate(int x) { MessageBox.Show(x.ToString()); });

            //Example 4 (equalent)
            setEventHandler(goodbyeDelegatesAndLambdas);
        }

        void goodbyeDelegatesAndLambdas(int x)
        {
            MessageBox.Show(x.ToString());
        }
    }
}