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CHAPTER 26 OTHER USEFUL FEATURES AND CLASSES
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// try to parse a string that doesn't contain a numeric value string myBadString = "Hello"; try { int myBadInt = Convert.ToInt32(myBadString); } catch (FormatException) { Console.WriteLine("Cannot parse {0} to an int", myBadString); } // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } In Listing 26-1, I perform a series of different type conversions using the Convert class. If a conversion can be performed, the ToXXX method returns the value specified by the parameter by using the target type. You can see this happening when I convert from an int to a long:
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long myLong = Convert.ToInt64(myInt);
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If the value cannot be represented using the target type, then a System.OverflowException will be thrown, just as we saw with manual overflow checking in 5. You can see this when I try to convert an int value that is too large to represent as a byte: try { byte myByte = Convert.ToByte(myInt); } catch (OverflowException) { Console.WriteLine("Got an OverflowException converting to a byte"); } When you pass a string value to a ToXXX method, the Convert class parses the string to create a value using the target type. If the string can t be parsed, then a System.FormatException is thrown. You can see successful and unsuccessful attempts to parse a string to a float and an int, respectively, in Listing 26-1. Compiling and running Listing 26-1 produces the following output: Long value: 2147483647 Got an OverflowException converting to a byte Int value: 23 Float value: 23 Cannot parse Hello to an int Press enter to finish
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Returning Multiple Results from Methods Using Tuples
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There are times where you want to return multiple objects or values as results from a method. We covered creating output parameters using the out keyword in 9, which would let us create a method like this:
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CHAPTER 26 OTHER USEFUL FEATURES AND CLASSES
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public void int out out
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PerformCalculation(int firstValue, secondValue, int sumResult, int productResult) {
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// calculate the sum int sum = firstValue + secondValue; // calculate the product int product = firstValue * secondValue; // pass the results back via the out parameters sumResult = sum; productResult = product; } Output parameters work very nicely, but they require a slightly odd syntax when they are called. The variables that will be used to hold the results have to be defined in advance, and the output parameters have to be marked with the out keyword in the method call as well, like this: int sum, product; PerformCalculation(10, 20, out sum, out product); Another approach would be to create a custom type to contain the various objects or values. We could do this using a struct or class, like this: public struct CalculationResult { public int SumResult; public int ProductResult; } public CalculationResult PerformCalculation(int firstValue, int secondValue) { // calculate the sum int sum = firstValue + secondValue; // calculate the product int product = firstValue * secondValue; // pass the results back via the struct return new CalculationResult() { SumResult = sum, ProductResult = product }; } This feels more natural to me. Using parameters to get results always feels...wrong. It is a very effective technique, but parameters will forever be inputs to me. By creating a struct to contain the results, we avoided using output parameters and got to the same solution. Here s how you would call a method like this:
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CalculationResult result = PerformCalculation(10, 20);
CHAPTER 26 OTHER USEFUL FEATURES AND CLASSES
If you want to get this effect but don t want to create a custom type for each result like this you need to return, you can use the System.Tuple class. This is a handy little class that you can use to create generic Tuple objects that contain multiple objects or values. Listing 26-2 provides a demonstration. Listing 26-2. Using the Tuple Class using System; class Calculator { public Tuple<int, int> PerformCalculation(int firstValue, int secondValue) { // calculate the sum int sum = firstValue + secondValue; // calculate the product int product = firstValue * secondValue; // create and return a tuple<T, T> to contain the result return Tuple.Create(sum, product); } } class Listing 02 { static void Main(string[] args) { Tuple<int, int> result = new Calculator().PerformCalculation(10, 20); Console.WriteLine("Sum result: {0}, Product result: {1}", result.Item1, result.Item2); // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } You create tuples using the static Create method of the Tuple class. There are overloaded versions that take from one to eight parameters. The Create method returns a generic Tuple object that is strongly typed based on the parameters you passed to the Create method. For example, when I called the Create method with two int values, the result was a Tuple<int, int>, which you can see is the result type from the PerformCalculation method in Listing 26-2. You can access the objects or values in the tuple using the properties called Item1, Item2, Item3, and so on, where the number of properties corresponds to the number of parameters you passed to the Tuple.Create method. I tend to use the Tuple class when I am quickly prototyping a solution and don t yet have a firm idea of how the results will be passed around. They are a quick and convenient way of compositing complex results. I avoid using them in methods that other programmers will use, preferring to define a custom type instead.
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