C# 4 and .NET 4

300 ❘ ChaPTer 12 dynAmic lAnGuAGe extensiOns
The CallSite is a type that handles the lookup at runtime. When a call is made on a dynamic object at
runtime, something has to go and look at that object to see if the member really exists. The call site caches
this information so the lookup doesn’t have to be performed repeatedly. Without this process, performance
in looping structures would be questionable.
After the CallSite does the member lookup, the CallSiteBinder is invoked. It takes the information from
the call site and generates an expression tree representing the operation the binder is bound to.
There is obviously a lot going on here. Great care has been taken to optimize what would appear to be a
very complex operation. It should be obvious that while using the dynamic type can be useful, it does
come with a price.
hosTing The dlr sCriPTrunTime
Imagine being able to add scripting capabilities to an application. Imagine passing values in and out
of the script so the application can take advantage of the work that the script does. These are the kind of
capabilities that hosting the DLR’s ScriptRuntime in your app gives you. Currently, IronPython, IronRuby,
and JavaScript are supported as hosted scripting languages.
With the ScriptRuntime, you have the capability of executing snippets of code or a complete script stored
in a file. You can select the proper language engine or allow the DLR to figure out which engine. The script
can be created in its own app domain or in the current one. Not only can you pass values in and out of the
script, but you can call methods on dynamic objects created in the script.
With this amount of flexibility there are countless uses for hosting the
ScriptRuntime. The following example demonstrates one way that
the ScriptRuntime can be used. Imagine a shopping cart application.
One of the requirements is to calculate a discount based on certain
criteria. These discounts change often as new sales campaigns are
started and completed. There are many ways of handling such a
requirement; this example shows how it could be handled using the
ScriptRuntime and a little Python scripting.
For simplicity, the example is a Windows client app. It could be part of
a larger web application or any other application. Figure 12-1 shows a
sample screen for the application.
The application takes the number of items and the total cost of the items
and applies a discount based on which radio button is selected. In a real
application the system would use a slightly more sophisticated way to
determine the discount to apply, but for this example, the radio buttons
will work.
Here is the code that performs the discount:
figure 12-1
private void button1_Click(object sender, RoutedEventArgs e)
{
string scriptToUse;
if (CostRadioButton.IsChecked.Value)
{
scriptToUse = "AmountDisc.py";
}
else
{
scriptToUse = "CountDisc.py";
}
ScriptRuntime scriptRuntime = ScriptRuntime.CreateFromConfiguration();
ScriptEngine pythEng = scriptRuntime.GetEngine("Python");
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