how to change type from "string" to "ObjectId" in C#

[new player]

excuse me.i want to use "_id" , but i don't know how to change the
type from "string" to "ObjectId" in C#.

thank you.

Code:
var query = new QueryDocument("_id", ? );

[Robert Stam]

You can use ObjectId.Parse. For example:

var s = "4d6c5246e447ad3bb4495c17";
var query1 = new QueryDocument("_id", ObjectId.Parse(s));
var query2 = Query.EQ("_id", ObjectId.Parse(s));

I've also illustrated the Query builder in case you prefer to use it
instead of building the QueryDocument by hand.

[new player]

it's work! thanks!!!