AN OVERVIEW OF .NET ERROR HANDLING in Font

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CHAPTER 4 AN OVERVIEW OF .NET ERROR HANDLING
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Listing 4-3. Classes and Structures Can Look Similar. VB .NET Class MyExceptionTemplate1 Public Message As String Public ErrorNumber As Int16 End Class Structure MyExceptionTemplate2 Public Message As String Public ErrorNumber As Int16 End Structure C# class MyExceptionTemplate1 { public string Message; public Int16 ErrorNumber; } struct MyExceptionTemplate2 { public string Message; public Int16 ErrorNumber; } While there are differences in the way .NET treats classes and structures internally, if your template is simply holding data, then you can choose either one. Again, we will talk more about the differences between them in 6.
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While it is possible to use the code you type into a class or structure directly, most of the time they are just used as templates for making objects. You can think of each object you make as having a copy of the template code, but these objects each have their own space in memory. For example, to create an object from either the class or structure you just looked at, you would type in the following code: VB .NET Dim objMyCustomExceptionA As New MyExceptionTemplete1 Dim objMyCustomExceptionB As New MyExceptionTemplete2 C# MyExceptionTemplate1 objMyCustomExceptionA = new MyExceptionTemplate1(); MyExceptionTemplate2 objMyCustomExceptionB = new MyExceptionTemplate2();
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CHAPTER 4 AN OVERVIEW OF .NET ERROR HANDLING
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Once your objects, objMyCustomExceptionA and objMyCustomExceptionB, are made in memory, you can fill them up with data. When we created the templates, we designed the Message and ErrorNumber variables as places to hold the data for each object. Since each object is just a copy of the class that is its template, you can make one or more copies as needed. Each will have its own space in memory and each can hold different data. Changing the values of one object will not affect another. Nor will it affect the class or structure at all, because their code is held in their own memory spaces separate from the objects you create from them. When you work with the data in an object, like the Message and ErrorNumber variables here, most programmers refer to these as an object s properties. This may be familiar to you since, by now, you have set the Text property of a Textbox object many times, and a Textbox object is one made from Microsoft s Textbox class. You set and read the properties of the objects you create just as you would set and read the properties of a Textbox object. VB .NET objMyCustomExceptionA.Message = "Your Friendly Message here" objMyCustomExceptionA.ErrorNumber = 123 MessageBox.Show(objMyCustomException.Message) C# objMyCustomExceptionA.Message = "Your Friendly Message here"; objMyCustomExceptionA.ErrorNumber = 123; MessageBox.Show(objMyCustomException.Message);
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Microsoft made a number of predefined classes for common errors. These are collectively referred to as the Exception classes. When .NET comes across an error, like trying to divide by zero, the .NET Framework automatically creates an object in memory based on the appropriate Exception class. If your code includes a Catch statement in its Try-Catch block, you can include a variable in the Catch statement that will allow you to access the Exception object that was created. Since variables act as named placeholders that point to a memory address, this variable will then be linked to the address of the object created by the .NET Framework when that type of error happens. For example, in Listing 4-4, the ex variable will be mapped to an object made from the DivideByZeroException class if you set the value of Gallons to zero and try to divide Distance by Gallons. Listing 4-4. Typical Try-Catch Block VB .NET Try 'Add code to set these values from the UI Distance = txtDistance.Text Gallons = txtGallons.Text Mileage = Distance / Gallons Catch ex As DivideByZeroException MessageBox.Show(ex.Message)
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CHAPTER 4 AN OVERVIEW OF .NET ERROR HANDLING
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Catch ex as Exception MessageBox.Show(ex.Message) End Try C# try { Distance = txtDistance.Text; Gallons = txtGallons.Text; Mileage = Distance / Gallons; } catch (DivideByZeroException ex) { MessageBox.Show(ex.Message); } catch (Exception ex) { MessageBox.Show(ex.Message); } The ex variable automatically points to the object created when the error happened, thanks to the way .NET handles errors. However, .NET can only point a variable to an Exception object if the data type of the variable, in this case ex, is compatible with the error object. For example, if an Exception object is made from the DivideByZeroException class and you create a Catch clause that has the same data type (as in Catch ex As DivideByZeroException), then .NET considers this a match, and the code for that particular Catch block runs. As it turns out, Microsoft allows the data type of Exception to point to any of the Exception objects. So, if your Catch statement does not specify an exception type, or uses the Exception class as the data type, then any exception is considered a match. That is why it s important that these generic versions should be the last Catch statement of that Try-Catch block; otherwise, it will match any exception that you did not catch in the preceding Catch blocks. Once a variable, like ex in our example, points to an object, it will now be able to access the properties Microsoft created in that Exception class, such as its Message property.
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