StartsWithAnyOf extension method

Here comes another great string extension for improved readability. You all know how to check if a string starts with another string (yep, it's the StartsWith method I'm talking about).

But what if you wan't to check if a string starts with any of a series of strings? Well, you'd have to do something like this.

string name = "Mr. Markus Olsson"
var l = new List<string> { "Dr", "Mr", "Ms" };
bool found;
foreach(string s in l) {
    if(name.StartsWith(l)) {
        found = true;
        break;
    }
}

Or, you could use lambdas for a much more elegant solution

string name = "Mr. Markus Olsson"
var l = new List<string> { "Dr", "Mr", "Ms" };
bool found = l.Exists(prefix => name.StartsWith(prefix));

That's pretty cool, right? The .Exists method on the list object takes a Predicate as a parameter and executes that predicate with each element as it's first argument until it finds a match. Coolio indeed but we can do better readability wise.

Enter StartsWithAnyOf extension methods

/// <summary>
/// Checks to see if the string starts with any of the supplied strings
/// </summary>
/// <param name="s">The string to check for a start value</param>
/// <param name="strings">One or more strings</param>
/// <returns>True the strings starts with any of the supplied strings, false otherwise</returns>
public static bool StartsWithAnyOf(this string s, params string[] strings)
{
    if (s == null)
        throw new ArgumentNullException("s");

    if (strings == null)
        throw new ArgumentNullException("strings");

    if (strings.Length == 0)
        throw new ArgumentOutOfRangeException("strings", "You must supply one or more strings");

    return Array.Exists(strings, (prefix => s.StartsWith(prefix)); 
}

/// <summary>
/// Checks to see if the string starts with any of the supplied strings
/// </summary>
/// <param name="s">The string to check for a start value</param>
/// <param name="strings">One or more strings</param>
/// <returns>True the strings starts with any of the supplied strings, false otherwise</returns>
public static bool StartsWithAnyOf(this string s, List<string> strings)
{
    if (s == null)
        throw new ArgumentNullException("s");

    if (strings == null)
        throw new ArgumentNullException("strings");

    if (strings.Count == 0)
        throw new ArgumentOutOfRangeException("strings", "You must supply one or more strings");

    return strings.Exists(x => s.StartsWith(x));
}

This allows us to rewrite our code to

string name = "Mr. Markus Olsson"
bool found = name.StartsWithAnyOf("Dr.", "Mr.", "Ms.");

Readability in a nutshell. Variations of these extension methods includes an override that takes a StringComparison in order to allow for case insensitive lookup. The EndsWithAnyOf method is of course also a must.

Update: as mattias pointed out there's a bit of code duplication here but that's intentional, read why

Update 2: I changed my mind again after discussing it with mattias and for the sake of readability and code-duplication I've decided to wrap the first method into a call of the second.

Update 3: James Curran pointed out that the Array class have an Exist method and that's of course what you want for the string array. Thanks James!

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