C# Passing array with specific array index

Question

I want to pass an array to a function with specific index in C#, like we do in C++, something like below:

void foo(int* B) {
// do something;
}

int main() {
int A[10];
foo(A + 3);
}

In this case suppose A starts with base address 1000, then B would be 1000+(sizeof(int)*3). The answer in this link uses Skip() function call but in this case B should be of type IEnumerable<int> and on calling Skip() with ToArray() it creates a copy with different base address, but I want to pass this same array to function foo. How can we do that in C#?

Why not simply pass the array and the offset? Yes, that is one extra parameter, but is that such a big deal? Why overcomplicate things? Maybe even the C++ code would be better off doing things that way, to be honest. — PaulMcKenzie
– PaulMcKenzie, Commented Apr 8, 2022 at 13:51
@PaulMcKenzie That's quite a big deal. That is the opposite of dependency injection. Now every array argument is two. The C++ code would preferably be a std::span. — Passer By
– Passer By, Commented Apr 8, 2022 at 13:54
I think you may be looking for “span”. It passes part of an array, but doesn’t make a copy. You may have only 1 element in your span, but I think it gives you what you want. — Mike Wodarczyk
– Mike Wodarczyk, Commented Apr 8, 2022 at 13:59
^^ All About Span: Exploring a New .NET Mainstay & Span<T> struct — Fildor
– Fildor, Commented Apr 8, 2022 at 14:00
Note that you can use pointers in c# if you allow unsafe code, but span<T> is probably a better option in the vast majority of cases. — JonasH
– JonasH, Commented Apr 11, 2022 at 6:59

Douglas Ferreira · Accepted Answer · 2022-04-08 14:30:23Z

2

This has been answered here: C# Array slice without copy

You can use ArraySegment or Span to achieve this

public static void Main()
{
    int[] arr1 = new int[] {2, 5, 8, 10, 12};
    int[] arr2 = new int[] {10, 20, 30, 40};

    DoSomethingWithSegmentReference(new ArraySegment<int>(arr1, 2, arr1.Length - 2));
    
    DoSomethingWithSpanReference(arr2.AsSpan().Slice(1, arr2.Length - 1));
}

private static void DoSomethingWithSegmentReference(ArraySegment<int> slice){
    for(int i = 0; i < slice.Count; i++)
        slice[i] = slice[i] * 2;
}

private static void DoSomethingWithSpanReference(Span<int> slice){
    for(int i = 0; i < slice.Length; i++)
        slice[i] = slice[i] * 2;
}

answered Apr 8, 2022 at 14:30

Douglas Ferreira

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3 Comments

Fildor Over a year ago

If you think this has been answered already, please mark it as duplicate instead.

Douglas Ferreira Over a year ago

I think he asked the same thing in a totally different way to think. I think his question is biased to a lower level lang like C and C++, thinking in passing pointers, memory address offsets and stuff. But in higher level langs such as C#, Golang, Python, JS, is quite common have classes or methods for array slicing he just didn't know that

dragon Over a year ago

Thanks Douglas, I think among ArraySegment and Span, the Span more fits in my requirements...

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