扩展方法未分配值

Question

I have a custom sort method for my dataset. I call it like this:

ds.Sort("column");

where ds is of type dataset. Then I sort the first table (sorting code itself is not relevant as the breakpoint shows the result is correct). My sort method looks like this:

public static void Sort(this DataSet ds, string column)
{
    DataSet newDs = ds.Copy();

    //sorting occurs

    ds = newDs.Copy();
} // <- breakpoint

At the place of breakpoint both ds and newDs have everything sorted like it should. However once I move forward and get to the line after calling this sort method, ds is no longer sorted. I also tried the same with simply doing

ds.Clear(); 

in that Sort method and this time it worked. Am I not allowed to assign a value to the object? Is it possible to do this in some way?

Answer 1

Am I not allowed to assign a value to the object?

You're not assigning a value to an object . You're assigning a value to a variable ... and that variable is a parameter. Changes to that parameter value are not seen outside the method, as C# uses pass-by-value by default. So for example, consider this code:

static void Foo(string input)
{
    input = "In Foo";
}
...
string text = "hello";
Foo(text);
Console.WriteLine(text); // Still prints "hello"

If you don't understand that, read my article on parameter passing for more details.

You probably want something like this:

public static DataSet CopyAndSort(this DataSet ds, string column)
{
    DataSet newDs = ds.Copy();

    //sorting occurs
    return newDs; // No need to copy it *again*.
}

Then you can use:

ds = ds.CopyAndSort("column");

Answer 2

The difference between

ds.Clear();

and

ds = newDs.Copy();

is that the first operates on the DataSet object that you have passed in through a reference ds , while the second one re-assigns a new object to that variable, making the original object inaccessible to your extension method.

You can use Merge method as a backdoor for filling the original data set with sorted data:

ds.Reset();
ds.Merge(newDs);

This will work, because the data would be copied into the original DataSet .