/// <summary>
/// This is used to get the string result from evaluating an XPath expression against the given document
/// and a context created from a set of key/value pairs.
/// </summary>
/// <param name="document">The document to use</param>
/// <param name="expression">The XPath expression to evaluate</param>
/// <param name="keyValuePairs">A set of key/value pairs to use when creating the context</param>
/// <returns>The evaluated expression result</returns>
/// <example>
/// <code language="cs">
/// string result = document.EvalXPathExpr("concat($key, '.htm')", "key", "filename");
/// </code>
/// </example>
/// <exception cref="ArgumentException">This is thrown if the <paramref name="keyValuePairs"/>
/// parameter contains an odd number of parameters.</exception>
public static string EvalXPathExpr(this IXPathNavigable document, XPathExpression expression,
params string[] keyValuePairs)
{
if (keyValuePairs.Length % 2 != 0)
{
throw new ArgumentException("There must be a value for every key name specified", "keyValuePairs");
}
CustomContext cc = new CustomContext();
for (int i = 0; i < keyValuePairs.Length; i += 2)
{
cc[keyValuePairs[i]] = keyValuePairs[i + 1];
}
return(document.EvalXPathExpr(expression, cc));
}