Let's take a quick look at how the WikiTalk Substring method is implemented for extracting part of a string:

 [ExposedMethod(ExposedMethodFlags.CachePolicyNone, "Answer a substring of this string ...")]
 public string Substring(ExecutionContext ctx, int index, [ExposedParameter(true)] int length)
 {
    if (ctx.TopFrame.WasParameterSupplied(2))
        return Value.Substring(index, length);
    else
        return Value.Substring(index);
 }

This is an example of a method that requires an ExecutionContext. After this parameter, the rest of the parameters are visible in WikiTalk. In this case, there are two parameters in WikiTalk for Substring: index and length.

By default in WikiTalk, all parameters are required. In this case, though, the parameter length parameter is marked as optional with [ExposedParameter(true)].

In the implementation of this method, the code checks to see if the parameter was supplied:

    if (ctx.TopFrame.WasParameterSupplied(2))

From then on, the code can do whatever it thinks is reasonable if the parameter is not supplied...