.NET Tip: Converting Strings to Numbers

Tuesday Oct 16th 2007 by Jay Miller

Converting strings of digits into numeric data types may not always yield the result you expect. Take a look at three ways to convert a string of digits to an integer and learn what you need to watch out for with each method.

.NET Tip: Converting Strings to Numbers

First, take a look at the Convert class. The Convert class offers methods to convert to and from every base data type, but for the purpose of this tip I'm only going to use the Convert.ToInt32() method. The method accepts a string parameter and returns an integer. If you pass in a string containing a valid integer value, the method returns an integer as expected. If you pass in a null value, the value 0 is returned. If you pass in a string that cannot be converted to an integer, an exception is raised. Here are examples of calls to the Convert.ToInt32() method with the comment indicating the result.

int x; 

x = Convert.ToInt32("123");    // x = 123
x = Convert.ToInt32(null);     // x = 0
x = Convert.ToInt32("abc");    // System.FormatException

Another option is to use the Parse() method available on the numeric base data types. For my purpose, I'm going to stick with int.Parse() for my examples. As with Convert.ToInt32(), if you pass in a string containing a valid integer value, the method returns an integer as expected. Passing in null or a string that cannot be converted, however, results in System.ArgumentNullException and System.FormatException exceptions respectively. Here is how you would use int.Parse().

int x; 

x = int.Parse("123");    // x = 123
x = int.Parse(null);     // System.ArgumentNullException
x = int.Parse("abc");    // System.FormatException

Until very recently I almost always used various flavors of Convert to convert strings to numbers. I avoided using Parse() mainly because of how null values are handled. In my application, having the result be 0 instead of an exception was preferred. I recently came across the TryParse() method on numberic base data types and have begun using it. The difference with TryParse() is that it takes the variable to store the result of the parsing as an out parameter and returns true or false to indicate whether the parsing was successful. No exceptions are raised for null values or values that cannot be converted. For most applications, that is how I prefer to have those conditions handled. Below are examples of using TryParse().

int x;
bool b;

b = int.TryParse("123", out x);    // x = 123, b = true
b = int.TryParse(null, out x);     // x = 0,   b = false
b = int.TryParse("abc", out x);    // x = 0,   b = false

Depending upon your application, one of these methods of converting strings to numbers may have an advantage over the others. I just want to make you aware of how the different options behave so you are able to make good decisions when designing your application.

About the Author

Jay Miller is a Software Engineer with Electronic Tracking Systems, a company dedicated to robbery prevention, apprehension, and recovery based in Carrollton, Texas. Jay has been working with .NET since the release of the first beta and is co-author of Learn Microsoft Visual Basic.Net In a Weekend. Jay can be reached via email at jmiller@sm-ets.com.

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