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By the end of this chapter, you will be able to Define the difference between syntax, runtime, and logic errors Explain how to use and modify breakpoints Explain how to use the debugging windows Explain how Try-Catch blocks work Explain what the Exception class is Create code to Throw Exception errors Use Debug and Trace to get debugging information
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CHAPTER 4 AN OVERVIEW OF .NET ERROR HANDLING
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Syntax, Runtime, and Logic Errors
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Errors that occur in a program are often called exceptions. Exceptions can be broken down into three basic categories: syntax, runtime, and logic errors. The syntax exceptions are the most commonly created and easiest to fix of these three. Let s take a quick look at some things you need to know about syntax errors before you look at the other two.
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Syntax exceptions are the errors you make when you type in a keyword incorrectly or you forget to use a symbol (like a parenthesis): MessageBox.Show TextBox1.Text 'Wrong MessageBox.Show = TextBox1.Text 'Wrong MessageBox.Show (TextBox1.Text) 'Right These can be easily fixed if you know the correct way they should have been typed. However, if you re not sure about what the code should look like, they can be a real pain. VS .NET lets you know when you have made a syntax error by underlining the code causing the error. It also gives you information about the error when you hover over it with your mouse cursor. In addition, Visual Studio 2005 now includes an Error List window (errors were shown in the Task List window before this). You can open this new window by going to the menu at the top of Visual Studio and choosing View Error List. Although having your errors pointed out is helpful, sometimes the error messages you receive are not unless you already know some of the terms used by Microsoft s developers. For example, here is a simple error message that is easy to understand. The errors shown in Figure 4-1 indicate that a parenthesis is missing. It also tells you that the statement has not been ended yet.
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Figure 4-1. A simple error message Fixing this exception is just a matter of adding the missing parenthesis to complete the statement, at which time both of these errors will disappear from the Error List. Now let s look at a more cryptic error message, as shown in Figure 4-2. This message makes sense if you already know what an overloaded method is, but it is much less helpful if you don t have a clue, or you cannot quite remember what that means. In those cases, you need to translate the message into individual terms to see if you can figure out what it is trying to tell you.
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CHAPTER 4 AN OVERVIEW OF .NET ERROR HANDLING
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Figure 4-2. A more complex error Starting with the first term overloaded, an overloaded method is where you create two or more methods (method is a generic term for a function or subroutine) with the exact same name, but each has its own unique set of parameters. You can think of this as having just one method of that name, but with multiple versions of the way it works. Here is an example: VB .NET Public Function RunTest() As String 'Put some code here to do something End Function Public Function RunTest(ByVal Parm1 As String) As String 'Put some code here to do something 'this time using Parameter 1 End Function C# public string RunTest() { //Put some code here to do something } public string RunTest(string Parm1) { //Put some code here to do something //this time using Parameter 1 } In this code, the RunTest() function now has two versions of itself: one that has no parameters and the other with one parameter. This is commonly referred to in object-oriented programming (OOP) terms as overloading the method (we will take a closer look at these terms later in the book, specifically in 6). In .NET, Microsoft uses this same design choice in many places. For example, when you start typing out MessageBox.Show(), a ToolTip pops up in the code window and explains that there are 21 versions, or overloads, of the Show() method and what parameters they use (see Figure 4-3).
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Figure 4-3. ToolTip showing 21 versions of MessageBox.Show()
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