Question

I see some code like this:

textBox.TextChanged += (s, e) => this.Foo();

but I don’t know what are “s” and “e” ? what is the topic I should study in C# for this line of code?

Answer 1

They are the parameters of the lambda-function.

The compiler infers their types from the context, but it is allowed to write a longer (more informative) form:

 textBox.TextChanged += (object s, EventArgs e) => { this.Foo(); };

In this notation it is easier to see that they are method parameters.
On the other side of => is the method body.

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In response to comment:

Now is there a way to also rewrite the same lambda expression I have in a simpler C# syntax?

Yes, you can always use the classic notation. And while that may not be ‘better’ or even ‘simpler’ it is easier to understand when you are learning this.

// The setup method
void MyMethod()
{
   //textBox.TextChanged += new Eventhandler(MyTextChangedHandler);  // C#1 and later
   textBox.TextChanged += MyTextChangedHandler;                      // C#2 and later
}

// The subscribed method. The lambda is an inline version of this. 
private void MyTextChangedHandler(object s, EventArgs e)
{ 
   this.Foo(); 
}