Passing a sub array as an array in C#

Question

This is an easy one, I am sure, but I can't seem to find it on google.

If I have a function:

foo(int[] a) {}

and I want to call it with a subarray of an array:

for instance: myNums[3]+ (i.e. an array {4,5,6}), given myNums:
int[] myNums = { 1,2,3,4,5,6};

Of course, I don't want to create a new array of just those sub items. I certainly could just pass the array and the index, but then I would have to write all the function code with offsets...ugly.

So, how does one do this in C# (.NET 2.0)?

foo(&myNums[3]) is reporting unsafe.

foo(myNums[3]) won't work (it is passing just an int).

Do I need to map this to some kind of collection?

If it were plain C, foo(&(myNums[3])) would have been indeed correct, FWIW. — Mark Rushakoff
– Mark Rushakoff, Commented Jul 31, 2009 at 2:03

Peter Mortensen · Accepted Answer · 2009-12-26 19:23:04Z

4

Passing the address of an array to a function is something not supported by C# out-of-the-box. Either you have to do it using unsafe method, the code will be C-like (though you need to compile with unsafe option). or you would have to code it the way managed code are ought to be, in this case, pass the offset.

    public Form1()
    {
        InitializeComponent();
        int[] a = { 4, 5, 6, 7, 8 };

        unsafe
        {
            fixed (int* c = a)
            {
                SubArrayPointer(c + 3);
            }
        }
        SubArray(a, 3);
    }

    unsafe void SubArrayPointer(int* d)
    {
        MessageBox.Show(string.Format("Using pointer, outputs 7 --> {0}", *d));
    }

    void SubArray(int[] d, int offset)
    {
        MessageBox.Show(string.Format("Using offset, outputs 7 --> {0}", d[offset]));
    }

edited Dec 26, 2009 at 19:23

Peter Mortensen

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answered Jul 31, 2009 at 2:43

Michael Buen

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Comments

Pavel Minaev · Accepted Answer · 2009-07-31 01:54:47Z

3

If you have a function exactly as defined (i.e. it takes an array), there's no way you can do this.

If you'd have a function that would take some collection interface instead, e.g.:

void foo(IList<int> a) { ... }

then you could pass an array to it, and you could also write your own IList<T> implementation that wraps a subrange of any other IList<T> (but there isn't a stock solution for this in FCL).

answered Jul 31, 2009 at 1:54

Pavel Minaev

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1 Comment

teleball Over a year ago

I think this: "but there isn't a stock solution for this" is what I am looking for. That is too bad. Thanks.

Rex M · Accepted Answer · 2009-07-31 01:50:53Z

2

Use LINQ extension methods:

int[] myNums = { 1,2,3,4,5,6 };

var subset = myNums.Skip(2).Take(2); //will contain 3,4

int[] subsetAsArray = myNums.Skip(2).Take(2).ToArray();

This assumes you are using .NET 3.0+. You'll also need to include System.Linq in your code file.

answered Jul 31, 2009 at 1:50

Rex M

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2 Comments

Pavel Minaev Over a year ago

This creates a copy, and he is specifically asking for an alias.

Nader Shirazie Over a year ago

albahari.com/nutshell/linqbridge.aspx (support for linq added as a reference that works in .net 2.0)

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