AO的VBA开发－客户化界面及VBEditor的使用.ppt

Introduction to Programming ArcObjects with VBA 2-* Controlling the flow of an If Then statement If Then statements in Visual Basic can be more complex that those that simply evaluate a single condition. If needed, a programmer can evaluate numerous conditions in a single If Then block by using keywords such as ElseIf and Else. Inside an If Then block, execution of the entire procedure may also be terminated early by using Exit Sub. Exit Sub When the Exit Sub line of code is encountered, execution of the procedure is terminated immediately. Exit Sub will generally only appear inside of an If Then block in the context described below. Often, the purpose of an If Then statement in your Visual Basic code is to check for a condition that will cause a run-time error. If your code asks the user to input a currency amount, for example, you would want to verify that the input is numeric (e.g., 20.00 instead of $20.00, or twenty) before attempting to use it in a mathematical operation. If the value is not numeric, you need to stop execution of the procedure before an error occurs, as shown below. dblPrice = InputBox (Please enter a price (without a dollar sign or commas) ) If Not IsNumeric (dblPrice) Then ? check to see if the input is NOT numeric MsgBox Please enter only numbers!, vbExclaimation, Exiting Exit Sub ?**Stop execution of the procedure if the error-causing condition is true!** End If dblDiscount = dblPrice * 0.20 ? calculate a 20% discount on the price, this line will only execute if dblPrice is numeric Introduction to Programming ArcObjects with VBA 2-* Select Case In addition to the If Then branching construct, a Visual Basic programmer can also use a Select Case statement to make decisions in his or her code. Unlike an If Then statement, Select Case makes decisions based on the value of a variable instead of using Boolean expressions. These values do not have to be numeric; a Select Case statement could also use variables that contain strings, dates,