Check a string to see if all characters are hexadecimal values
<p>What is the most efficient way in C# 2.0 to check each character in a string and return true if they are all valid hexadecimal characters and false otherwise.</p>

<p>Example</p>

<pre><code>void Test()
{
    OnlyHexInString("123ABC"); // returns true
    OnlyHexInString("123def"); // returns true
    OnlyHexInString("123g"); // returns false
}
bool OnlyHexInString(string text)
{
    // Most efficient algorithm to check each digit in C# 2.0 goes here
}
</code></pre>
