C# in Depth

Expression trees243the tree and how parameters are bound. We start off by working out everything weneed to know about the method call that forms the body of the final expression B:the target of the method (in other words, the string we’re calling StartsWith on); themethod itself (as a MethodInfo); and the list of arguments (in this case, just the one).It so happens that our method target and argument will both be parameters passedinto the expression, but they could be other types of expressions—constants, theresults of other method calls, property evaluations, and so forth.After building the method call as an expression C, we then need to convert itinto a lambda expression D, binding the parameters as we go. We reuse the sameParameterExpression values we created as information for the method call: theorder in which they’re specified when creating the lambda expression is the order inwhich they’ll be picked up when we eventually call the delegate.Figure 9.3 shows the same final expression tree graphically. To be picky, eventhough it’s still called an expression tree, the fact that we reuse the parameter expressions(and we have to—creating a new one with the same name and attempting tobind parameters that way causes an exception at execution time) means that it’s not atree anymore.lambdaExpressionNodeType=LambdaType=System.BooleanBodyParameterscallMethodCallExpressionNodeType=CallType=System.BooleanlambdaParametersCollection ofParameterExpressionsMethodArgumentsObject(Contains)methodmethodArgstargetMethodInfo forstring.StartsWith(string)(Contains)Collection ofExpressionsParameterExpressionNodeType=ParameterType=System.StringName="x"methodArgParameterExpressionNodeType=ParameterType=System.StringName="y"(Contains)Figure 9.3 Graphical representation of expression tree that calls a method and uses parameters froma lambda expressionLicensed to Rhona Hadida