Checking if a string array contains a value, and if so, getting its position

var index = Array.FindIndex(stringArray, x => x == value)

We can also use Exists:

string[] array = { "cat", "dog", "perl" };

// Use Array.Exists in different ways.
bool a = Array.Exists(array, element => element == "perl");
bool c = Array.Exists(array, element => element.StartsWith("d"));
bool d = Array.Exists(array, element => element.StartsWith("x"));

EDIT: I hadn't noticed you needed the position as well. You can't use IndexOf directly on a value of an array type, because it's implemented explicitly. However, you can use:

IList<string> arrayAsList = (IList<string>) stringArray;
int index = arrayAsList.IndexOf(value);
if (index != -1)
{
    ...
}

(This is similar to calling Array.IndexOf as per Darin's answer - just an alternative approach. It's not clear to me why IList<T>.IndexOf is implemented explicitly in arrays, but never mind...)

Use System.Linq

stringArray.Contains(value3);

You can use Array.IndexOf() - note that it will return -1 if the element has not been found and you have to handle this case.

int index = Array.IndexOf(stringArray, value);

you can try like this...you can use Array.IndexOf() , if you want to know the position also

       string [] arr = {"One","Two","Three"};
       var target = "One";
       var results = Array.FindAll(arr, s => s.Equals(target));

IMO the best way to check if an array contains a given value is to use System.Collections.Generic.IList<T>.Contains(T item) method the following way:

((IList<string>)stringArray).Contains(value)

Complete code sample:

string[] stringArray = { "text1", "text2", "text3", "text4" };
string value = "text3";
if (((IList<string>)stringArray).Contains(value)) Console.WriteLine("The array contains "+value);
else Console.WriteLine("The given string was not found in array.");

T[] arrays privately implement a few methods of List<T>, such as Count and Contains. Because it's an explicit (private) implementation, you won't be able to use these methods without casting the array first. This doesn't only work for strings - you can use this trick to check if an array of any type contains any element, as long as the element's class implements IComparable.

Keep in mind not all IList<T> methods work this way. Trying to use IList<T>'s Add method on an array will fail.

string x ="Hi ,World";
string y = x;
char[] whitespace = new char[]{ ' ',\t'};          
string[] fooArray = y.Split(whitespace);  // now you have an array of 3 strings
y = String.Join(" ", fooArray);
string[] target = { "Hi", "World", "VW_Slep" };

for (int i = 0; i < target.Length; i++)
{
    string v = target[i];
    string results = Array.Find(fooArray, element => element.StartsWith(v, StringComparison.Ordinal));
    //
    if (results != null)
    { MessageBox.Show(results); }

}

I created an extension method for re-use.

   public static bool InArray(this string str, string[] values)
    {
        if (Array.IndexOf(values, str) > -1)
            return true;

        return false;
    }

How to call it:

string[] stringArray = { "text1", "text2", "text3", "text4" };
string value = "text3";
if(value.InArray(stringArray))
{
  //do something
}

You can try this, it looks up for the index containing this element, and it sets the index number as the int, then it checks if the int is greater then -1, so if it's 0 or more, then it means it found such an index - as arrays are 0 based.

string[] Selection = {"First", "Second", "Third", "Fourth"};
string Valid = "Third";    // You can change this to a Console.ReadLine() to 
    //use user input 
int temp = Array.IndexOf(Selection, Valid); // it gets the index of 'Valid', 
                // in our case it's "Third"
            if (temp > -1)
                Console.WriteLine("Valid selection");
            }
            else
            {
                Console.WriteLine("Not a valid selection");
            }