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Finding and Debugging Errors. Categories of Errors. Syntax errors are detected at compile time Use the Error List window to find these errors The debugging tools cannot help with syntax errors Runtime errors occur as an application executes
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Categories of Errors • Syntaxerrors are detected at compile time • Use the Error List window to find these errors • The debugging tools cannot help with syntax errors • Runtimeerrors occur as an application executes • Logicerrors cause an application to produce unexpected results but do not cause exceptions
Types of Runtime Errors • File processing errors • Arithmetic errors • Null reference errors • And many more
File Processing Errors • Sequential files with extra trailing carriage returns will generate errors • Sequential files with extra or missing fields will also cause errors • Omitting the carriage return on the last record may cause the last record to be ignored
Arithmetic Errors (1) • Numeric overflow results in the following exception: • Correct by performing double precision arithmetic instead of integer arithmetic
Arithmetic Errors (2) • Trying to convert an invalid value to a number will result in a System.FormatException • Call TryParse() to test before conversion
Arithmetic Errors (3) • Integer division by zero throws a System.DivideByZeroException: • Fix by testing the dividend value
Arithmetic Errors (4) • Division by zero is handled differently for floating point numbers and floating point division • Division by zero results in the value Infinity • Test using Double.IsInfinity(Value)
Null Reference Errors (Introduction) • Pure and simple, you reference an object with first creating an instance of that object • The following exception is thrown
Interrogating the Value of a Variable or Property • Do while the application is in break mode • In the Code Editor, highlight the variable, object, or property • Expand or collapse complex objects as necessary
Setting Breakpoints (1) • Use to temporarily suspend execution while a program runs • Multiple breakpoints may be set • Breakpoints may be enabled or disabled • It’s possible to clear all breakpoints • Breakpoints are persistent between invocations of Visual Studio
Setting Breakpoints (2) • Set simple breakpoints on a line by clicking in the left margin in the Code Editor • A maroon circle appears in the left margin • Breakpoints may be set only on executable statements because declaration statements don’t execute!
Setting Breakpoints (3) • Breakpoints appear in the Breakpoints window
Setting Conditional Breakpoints • Break when a condition becomes true or a value changes • Use for problems having long iteration counts
Setting Hit Count Breakpoints • Break on an iteration count • Hit count breakpoints are configurable Break after 999 iterations
Setting Conditional Breakpoints • Conditional breakpoints are hit when • The value of a Boolean condition is True • The value of a condition has changed
Setting Conditional Breakpoints (Illustration) • Set a conditional breakpoint
Setting WatchPoints • There are multiple Watch windows • Each works the same way • Just create an expression to be watched • Use Watchpoints instead of repeatedly typing the same expression in the Immediate window • Beware of setting too many Watchpoints – Execution will become quite slow
Setting WatchPoints (Illustration) • Watch the variable “f”
Other Debugging Windows (1) • The Autos window shows variables used in the current and previous statement
Other Debugging Windows (2) • The Locals window displays variables local to the current context
Other Debugging Windows (3) • Call stack shows the procedures and the order in which the procedures were called
Exceptions • Use to change the handling of exceptions when they occur