Why DataSet is being passed by reference without explicitly passing out or ref parameter? [duplicate]

Why DataSet is being passed by reference without explicitly passing out or ref parameter? [duplicate]

回答1

Strings are immutable, plus you are not modifying the string parameter (not that you can), but instead you are assigning a new reference to your parameter. The original remains in place.

With DataSet, you are modifying its contents and since it is a mutable reference type, you see the change in the caller.

Try the following in Main

    DataSet ds = new DataSet();
    ds.Tables.Add(new DataTable("tbl3"));
    FillDS(ds);

and then in FillDS assign a new reference to your DataSet like:

private static void FillDS(DataSet ds)
{
    ds = new DataSet(); //Here 
    ds.Tables.Add(new DataTable("tbl1"));
    ds.Tables.Add(new DataTable("tbl2"));
}

You will see that your DataSet still holds the old values and nothing is modified after calling FillDS

回答2

Both string and DataSet are reference types. So in both cases you pass references.

In the AssignString method, you don't change the string instance you passed, but assign a new instance ("new name") to the variable name. The fact that the name variable actually was a parameter does not matter (though it's considered a bad practice to re-assign parameters).

In FillDS you do not reassign a new instance, but access and manipulate the properties of the passed instance via ds.Tables.....

Possible to use dataset as ref

A DataSet is a Reference Type by nature. Value Types are primitive types like int, bool, double, long, etc.

DataSet is not the better approch to transfer data. You could use generics collections like List<T> and create a class (a DTO object for sample) which contains just the properties you need to bind into the form. With this you can get a better performance.

You could be sure if your query itno database to fill this dataSet is good query.

This article explain with some detail why is better using generics collections instead dataset. http://msdn.microsoft.com/en-us/magazine/cc163751.aspx