Null reference index

[DoctorMarvin21]

Return null if index is out of range:
var a = array1[?0];

If array has no element at index zero it will return null. It will work with reference types but could be expanded to value type if statement will return nullable:
int[] array1 = { 1, 3, 4, 5, 6 };
int? a = array1[?0]; // "a" is 1
int? b = array1[?5]; // "b" is null

Same could be done with ranges. If range exceeds array length it will return null:
int[] array1 = { 1, 3, 4, 5, 6 };
int[] array2 = array1[?0..2]; // { 1, 3 }
int[] array3 = array1[?0..5]; // null

This should help with situations when array is not null but index is out of range. With null-coalescing assignment this could be powerful tool for one line statements such as this one:
A a = array1[?5] ?? new A();

Using older syntax this could be achieved only with if-else statement.

[theunrepentantgeek]

I can't remember the last time I needed to put a guard statement around an array access to prevent out of bounds access and to provide a default value.

In my experience, attempting to access an out of bounds value in an array is virtually certain evidence of a bug. The remedy is to fix the bug, not to mask the problem by patching around the array index.

For the vanishingly small number of situations where this is desirable behaviour, a trivial extension method does what you want:

public static T? Index<T>(this T[] arr, int index)
    => index < arr.Length ? arr[index] : default(T);

Note that it doesn't need to use an if-else statement ... and it will work with any array.

[Joe4evr]

Also, there's a good chance the JIT could eliminate the bounds check of the array access since it sees the index has been checked against the length right before.

[canton7]

There's already a method for this: Enumerable.ElementAtOrDefault.

var b = array1.ElementAtOrDefault(10);

Yes you could argue it's a bit longer than ?, but 1) It already exists, 2) It doesn't normally need to be used very often, and 3) It stands out as being something unusual.

[DoctorMarvin21]

Yes, I forgot about this new LINQ feature and can agree with all of 3 statements.

[canton7]

new

It's been around since 2007!

[svick]

Keep in mind that ElementAtOrDefault does what it says: it gives you default if an element is not present, not null, which is different than what the proposed syntax does for value types.

Though you can work around that by using .Cast<T?> first.

[canton7]

That's true. Although, adding .Cast<T?>() means that ElementAtOrDefault becomes O(n), rather than being O(1) when used on collections which are indexable.

[TonyValenti]

I have a TryGet extension method for arrays that lets me treat them kind of like a dictionary.