C# casting, Indexof

Question

I'm new to programming (for 5 days). I wrote this code but index is always -1 (when 2 is expected) and I couldn't fix it.

My teacher wrote me just that:

Casting, Array.IndexOf(geburtsjahr, (int)1966);

I tried but still same.

This is my relevant code:

var geburtsjahr = new short[4] { 1998, 1992, 1966, 2006 }; 

// Expected: 2, Actual: -1
int index = Array.IndexOf(geburtsjahr, (int)1966);

Console.WriteLine(index); 

I tried to see right index number (should be 2).

Answer 1

It works if you do either of these:

(1)

var geburtsjahr = new short[4] { 1998, 1992, 1966, 2006 };
int index = Array.IndexOf(geburtsjahr, (short)1966);

(2)

var geburtsjahr = new int[4] { 1998, 1992, 1966, 2006 };
int index = Array.IndexOf(geburtsjahr, (int)1966); //or just 1966 without the cast.

The issue is that there is no int of any value in an array of short values, no matter what the numbers represent to us humans.

Answer 2

The main problem with your code is that you have short[] array and int value to find in it and .net can't do implicit cast from int to short for you. There are several overload Array.IndexOf implementations:

• Array.IndexOf(T[], T): will not be used, since .net can't cast int to short or short[] to int[] and come to a single generic T
• Array.IndexOf(Array, Object): this will be called

In case of Array.IndexOf(Array, Object) .net looks for an object (boxed int value) in an array of objects. The array contains short values (which can be boxed, but still short), not int, so .net can't find int.

Possible corrections:

1. Make geburtsjahr have int values:

var geburtsjahr = new int[4] { 1998, 1992, 1966, 2006 }; 

2. Search for short:

int index = Array.IndexOf(geburtsjahr, (short)1966);

3. Let .net do the cast for you (1966 will be of type short):

int index = Array.IndexOf<short>(geburtsjahr, 1966);