In some cases, you may not be able to define an expression's variables when it is compiled. In such cases, you can use the on-demand variables. If an expression references a variable that doesn't exist in the expression's variables, Flee will raise the following two events.
If the ResolveVariableType event is handled and a type is specified, Flee will then raise the ResolveVariableValue event every time the expression is evaluated. In that event, you have access to the variable's name and type and it is up to you to provide a compatible value.
The following example uses on-demand variables to ask the user for the value of each variable in the expression.
public static void Example()
{
ExpressionContext context = new ExpressionContext();
context.Variables.ResolveVariableType += new EventHandler<ResolveVariableTypeEventArgs>(GetVariableType);
context.Variables.ResolveVariableValue += new EventHandler<ResolveVariableValueEventArgs>(GetVariableValue);
string expression = "(a + b) * 1.5";
IDynamicExpression e = context.CompileDynamic(expression);
object result = e.Evaluate();
Console.WriteLine("The result is: {0}", result);
}
private static void GetVariableType(object sender, ResolveVariableTypeEventArgs e)
{
e.VariableType = typeof(double);
}
private static void GetVariableValue(object sender, ResolveVariableValueEventArgs e)
{
Console.Write("Enter value for variable '{0}' ({1}): ", e.VariableName, e.VariableType.Name);
string value = Console.ReadLine();
e.VariableValue = Convert.ChangeType(value, e.VariableType);
}
As you can see that two event handlers are defined, the GetVariableType specifies the type for a variable and GetVariableValue will ask the user to specify the value for that particular variable.
Let's execute the above code, and you will see the following output.
Enter value for variable 'a' (Double): 3.5
Enter value for variable 'b' (Double): 4.0
The result is: 11.25