public static void Copy(string sourceDirName, string destDirName, bool excludeSubdirectories)
{
    // Style #1: fluent
    Guard.MethodArgument("sourceDirName").IsNotNullOrEmpty(sourceDirName);
    Guard.MethodArgument("destDirName").IsNotNullOrEmpty(destDirName, "The destination directory is required!");
 
    // Style #2: lambdas and expressions
    Guard.Argument.IsNotNullOrWhiteSpace(()=>sourceDirName);
    Guard.Argument.IsNotNullOrWhiteSpace(()=>destDirName, "The destination directory is required!");