c# - Why pass func<T> to constructor rather than T? -

i came across accepted answer of question about dealing datetime.now in unit tests contains following code example:

private readonly func<datetime> _nowprovider; public someclass(func<datetime> nowprovider) {     _nowprovider = nowprovider; }  public bool foo() {     return (_nowprovider().dayofweek == dayofweek.sunday); } 

instantiated such:

var s = new someclass(() => datetime.now); 

i've not used func<t> in c# thought i'd take at microsoft documentation it has following remarks:

you can use delegate represent method can passed parameter without explicitly declaring custom delegate. encapsulated method must correspond method signature defined delegate. means encapsulated method must have no parameters , must return value.

why more beneficial in example pass func<datetime>, instantiated class(() => datetime.now) constructor

rather pass in datetime parameter instantiated class(datetime.now) constructor?

according microsoft documentation mentioned above linq lambda constructors take func<t> arguments , experience these proves extremely flexible can't understand why?

rather pass in datetime parameter instantiated class(datetime.now) constructor?

because value should current datetime , not 1 when class has been instanciated.

when code runs, func returns date of when code executed.

if datetime stored in field, time of creation, not now.

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i have example.

let's create instance of class @ 23:59:55 on saturday.

10 seconds later, following snipped:

(passeddatetime.dayofweek == dayofweek.sunday);  

would return false.

with provider, datetime on sunday - time executed at.

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technical:

datetime struct.

when passing datetime method or constructor parameter, passed value, not reference.

thus datetime not date, snapshot of value.

you can confirm yourself:

var datetime = datetime.now;  system.threading.sleep(1000); bool equals = datetime == datetime.now; // false