Exercise - Examine the TryParse() method

  • 12 minutes

When working with data, sometimes, you need to convert string data into a numeric data type. As you learned in the previous unit, because the string data type can hold a non-numeric value, it's possible that performing a conversion from a string into a numeric data type causes a runtime error.

For example, the following code:

string name = "Bob";
Console.WriteLine(int.Parse(name));

Causes the following exception:

System.FormatException: 'Input string was not in a correct format.'

To avoid a format exception, use the TryParse() method on the target data type.

Use TryParse()

The TryParse() method does several things simultaneously:

  • It attempts to parse a string into the given numeric data type.
  • If successful, it stores the converted value in an out parameter, explained in following section.
  • It returns a bool to indicate whether the action succeeded or failed.

You can use the Boolean return value to take action on the value (like performing some calculation), or display a message if the parse operation was unsuccessful.

Note

In this exercise, you'll use the int data type, but a similar TryParse() method is available on all numeric data types.

Out parameters

Methods can return a value or return "void" - meaning they return no value. Methods can also return values through out parameters, which are defined just like an input parameter, but include the out keyword.

TryParse() a string into an int

  1. Delete or use the line comment operator // to comment out all of the code from the previous exercises.

  2. Update your code in the Visual Studio Code Editor as follows:

    string value = "102";
int result = 0;
if (int.TryParse(value, out result))
{
   Console.WriteLine($"Measurement: {result}");
}
else
{
   Console.WriteLine("Unable to report the measurement.");
}

  3. Examine this line of code:

    if (int.TryParse(value, out result))

    When calling a method with an out parameter, you must use the keyword out before the variable, which holds the value. The out parameter is assigned to the result variable in the code (int.TryParse(value, out result). You can then use the value the out parameter contains throughout the rest of your code using the variable result.

    The int.TryParse() method returns true if it successfully converted the string variable value into an int; otherwise, it returns false. So, surround the statement in an if statement, and then perform the decision logic, accordingly.

    The converted value is stored in the int variable result. The int variable result is declared and initialized before this line of code, so it should be accessible both inside the code blocks that belong to the if and else statements, as well as outside of them.

    The out keyword instructs the compiler that the TryParse() method doesn't return a value the traditional way only (as a return value), but also communicates an output through this two-way parameter.

    When you run the code, you should see the following output:

    Measurement: 102

Use the parsed int later in code

  1. To demonstrate that the result variable that was declared earlier, is populated by the out parameter and is also usable later in your code, update your code in the Visual Studio Code Editor as follows:

    string value = "102";
int result = 0;
if (int.TryParse(value, out result))
{
   Console.WriteLine($"Measurement: {result}");
}
else
{
   Console.WriteLine("Unable to report the measurement.");
}
Console.WriteLine($"Measurement (w/ offset): {50 + result}");

  2. On the Visual Studio Code File menu, select Save. The Program.cs file must be saved before building or running the code.

  3. In the EXPLORER panel, to open a Terminal at your TestProject folder location, right-click TestProject, and then select Open in Integrated Terminal. A Terminal panel should open, and should include a command prompt showing that the Terminal is open to your TestProject folder location.

  4. At the Terminal command prompt, to run your code, type dotnet run and then press Enter.

    Note

    If you see a message saying "Couldn't find a project to run", ensure that the Terminal command prompt displays the expected TestProject folder location. For example: C:\Users\someuser\Desktop\csharpprojects\TestProject>

    You should see the following output:

    Measurement: 102
Measurement (w/ offset): 152

  5. Examine the last line of code in the previous sample, Console.WriteLine($"Measurement (w/ offset): {50 + result}");, Since the result variable is defined outside of the if statement, it can be accessed later in your code.

Modify the string variable to a value that can't be parsed

Lastly, look at the other scenario - where the TryParse() is intentionally given a bad value that can't be converted into an int.

  1. Modify the first line of code, reinitialize the variable value to a different value.

    string value = "bad";

  2. Also, modify the last line of code to ensure that the result is greater than 0 before showing the second message.

    if (result > 0)
   Console.WriteLine($"Measurement (w/ offset): {50 + result}");

  3. The entire code example should now match the following code:

    string value = "bad";
int result = 0;
if (int.TryParse(value, out result))
{
   Console.WriteLine($"Measurement: {result}");
}
else
{
   Console.WriteLine("Unable to report the measurement.");
}

if (result > 0)
   Console.WriteLine($"Measurement (w/ offset): {50 + result}");

  4. Save your code file, and then use Visual Studio Code to run your code. You should get the following result:

    Unable to report the measurement.

  5. Examine the last two lines of code added in the previous sample.

    if (result > 0)
   Console.WriteLine($"Measurement (w/ offset): {50 + result}");

    Since result is defined outside of the if statement, result can be accessed later in your code outside of the code blocks. So then result can be checked for a value greater than zero before allowing result + offset to be written as output. Checking for a result value greater than zero avoids printing an offset value after the Unable to report the measurement. message.

Recap

The TryParse() method is a valuable tool. Here are few quick ideas to remember.

  • Use TryParse() when converting a string into a numeric data type.
  • TryParse() returns true if the conversion is successful, false if it's unsuccessful.
  • Out parameters provide a secondary means of a method returning a value. In this case, the out parameter returns the converted value.
  • Use the keyword out when passing in an argument to a method that has defined an out parameter.

Check your knowledge

1.

Which technique should be used to change myInput, a string value "2.71828", into a decimal variable myInputDecimal?

2.

Consider the C# code bool success = decimal.TryParse(input, out decimal number);. Which best describes the return type of decimal.TryParse()?