find if an integer exists in a list of integers

The way you did is correct. It works fine with that code: x is true. probably you made a mistake somewhere else.

List<int> ints = new List<int>( new[] {1,5,7}); // 1

---

List<int> intlist=new List<int>() { 0,2,3,4,1}; // 2

---

var i = 5;
var x = ints.Contains(i);   // return true or false

Here is a extension method, this allows coding like the SQL IN command.

public static bool In<T>(this T o, params T[] values)
{
    if (values == null) return false;

    return values.Contains(o);
}
public static bool In<T>(this T o, IEnumerable<T> values)
{
    if (values == null) return false;

    return values.Contains(o);
}

This allows stuff like that:

List<int> ints = new List<int>( new[] {1,5,7});
int i = 5;
bool isIn = i.In(ints);

Or:

int i = 5;
bool isIn = i.In(1,2,3,4,5);

As long as your list is initialized with values and that value actually exists in the list, then Contains should return true.

I tried the following:

var list = new List<int> {1,2,3,4,5};
var intVar = 4;
var exists = list.Contains(intVar);

And exists is indeed set to true.

If you just need a true/false result

bool isInList = intList.IndexOf(intVariable) != -1;

if the intVariable does not exist in the List it will return -1