Strings and Characters, Part 2 - 2.15 Converting Strings to Their Equivalent Value Type
(Page 3 of 11 )
Problem
You have a string that represents the equivalent value of a number ("12"), char ("a"), bool ("true"), or a color enumeration ("Red"). You need to convert this string to its equivalent value type. Therefore, the number "12" would be converted to a numeric value such as int, short, float, etc. The string "a" would be converted to a char value 'a', the string "true" would be converted to a bool value, and the color "Red" could be converted to an enumeration value (if an enumeration were defined that contained the element Red).
Solution Use the Parse static method of the type that the string is to be converted to. To convert a string containing a number to its numeric type, use the following code:
// This code requires the use of the System and System.Globalization namespaces
string longString = "7654321";
int actualInt = Int32.Parse(longString); // longString = 7654321
string dblString = "-7654.321"; double actualDbl = Double.Parse(dblString, NumberStyles.AllowDecimalPoint | NumberStyles.AllowLeadingSign); // longString = "-7654.321
To convert a string containing a Boolean value to a Boolean type, use the following code:
// This code requires the use of the System namespace
string boolString = "true"; bool actualBool = Boolean.Parse(boolString); // actualBool = true
To convert a string containing a char value to a char type, use the following code:
// This code requires the use of the System namespace
string charString = "t"; char actualChar = char.Parse(charString); // actualChar = 't'
To convert a string containing an enumeration value to an enumeration type, use the following code:
// This code requires the use of the System namespace
enum Colors
{
red, green, blue
}
string colorString = "blue";
// Note that the Parse method below is a method defined by System.Enum, not by Colors
Colors actualEnum = (Colors)Colors.Parse(typeof(Colors), colorString);
// actualEnum = blue
Discussion The static Parse method on certain types derived from the ValueType data types allows easy conversion from a string value to the value of that specific value type. The Parse method is supported by the following types:
Boolean Int64
Byte SByte
Decimal Single
Double UInt16
Int16 UInt32
Int32 UInt64
In addition to the Parse methods that take a single string parameter and convert it to the target data type, each numeric type has a second overloaded version of the Parse method that includes a second parameter of type System.Globalization. NumberStyles. This allows the Parse method to correctly handle specific properties of numbers, such as leading or trailing signs, decimal points, currency symbols, thousands separators, etc. NumberStyles is marked as a flag-style enumeration, so you can bitwise OR more than one enumerated value together to allow a group of styles to be used on the string.
The NumberStyles enumeration is defined as follows:
AllowCurrencySymbol
If the string contains a number with a currency symbol, it is parsed as currency; otherwise, it is parsed as a number.
AllowDecimalPoint
Allows a decimal point in the number.
AllowExponent
Allows the number to be in exponential notation format.
AllowHexSpecifier
Allows characters that specify a hexadecimal number.
AllowLeadingSign
Allows a leading sign symbol.
AllowLeadingWhite
Ignores any leading whitespace.
AllowParentheses
Allows parentheses.
AllowThousands
Allows group separators.
AllowTrailingSign
Allows a trailing sign symbol.
AllowTrailingWhite
Ignores any trailing whitespace.
Any
Applies any of the previous styles. This style simply ORs together all of the preceding styles.
Currency
Same as the All style, except that the AllowExponent style is omitted.
Float
Equivalent to AllowLeadingWhite | AllowTrailingWhite | AllowLeadingSign |AllowDecimalPoint | AllowExponent
HexNumber
Equivalent to AllowLeadingWhite | AllowTrailingWhite | AllowHexSpecifier
Integer
Equivalent to AllowLeadingWhite | AllowTrailingWhite | AllowLeadingSign
None
Applies none of the styles.
Number
Equivalent to AllowLeadingWhite | AllowTrailingWhite | AllowLeadingSign |AllowTrailingSign | AllowDecimalPoint | AllowThousands
If the NumberStyle parameter is not supplied when it is required (as when, for example, a numeric string includes a thousands separator), or if the NumberStyle enumeration is used on a string that does not contain a number in the supplied NumberStyle format, a FormatException exception will be thrown. If the size of the number in the string is too large or too small for the data type, an OverFlowException exception will be thrown. Passing in a null for the SourceString parameter will throw an ArgumentNullException exception.
The Parse method of the two non-numeric data types, bool and char, also deserve some additional explanation. When calling Boolean.Parse, if a string value contains anything except a value equal to the static properties Boolean.FalseString, Boolean. TrueString, or the string literals "false" or "true" (which are case-insensitive), a FormatException exception is thrown. Passing in a null for the SourceString parameter throws an ArgumentNullException exception.
When invoking char.Parse, if a string value containing more than one character is passed as its single argument, a FormatException exception is thrown. Passing in a null for the string parameter throws an ArgumentNullException exception.
The static Enum.Parse method returns an Object of the same type as specified in the first parameter of this method (EnumType). This value is viewed as an Object type and must be cast to its correct enumeration type.
This method throws an ArgumentException exception if the Value parameter cannot be matched to a string in the enumeration. An ArgumentNullException exception is thrown if a null is passed in to the Value parameter.
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