String.Format

With formatting, we change how data appears. The C# string.Format method helps—we use it to change how numbers are printed with format codes.

Format notes

We can place other text inside the formatting string to compose the entire result string. We can even change numbers to percentages.

First example

We use string.Format to combine 3 strings with formatting options. The first argument is the format string—it is a string literal.

using System;

// Part 1: variables.
string value1 = "cat";
int value2 = 10000;
DateTime value3 = new DateTime(2015, 11, 1);

// Part 2: use string.Format method with markers.
string result = string.Format("{0}: {1:0.0} - {2:yyyy}", value1, value2, value3);

// Part 3: write the result.
Console.WriteLine(result);
cat: 10000.0 - 2015

Number formats

We can specify that a value (like a double) can be formatted inside string.Format. A format string is the first argument to string.Format.

using System;

class Program
{
    static void Main()
    {
        // Format a ratio as a percentage string.
        // ... You must specify the percentage symbol.
        // ... It will multiply the value by 100.
        double ratio = 0.73;
        string result = string.Format("string = {0:0.0%}", ratio);
        Console.WriteLine(result);
    }
}
string = 73.0%

Padding

This can be expressed declaratively in format strings. Padding inserts characters at the left or right of the string to reach a specified length.

using System;

class Program
{
    static void Main()
    {
        // The constant formatting string.
        // ... It specifies the padding.
        // ... A negative number means to left-align.
        // ... A positive number means to right-align.
        const string format = "{0,-10} {1,10}";
        // Construct the strings.
        string line1 = string.Format(format, 100, 5);
        string line2 = string.Format(format, "Carrot", "Giraffe");
        // Write the formatted strings.
        Console.WriteLine(line1);
        Console.WriteLine(line2);
    }
}
100                 5
Carrot        Giraffe

ToString

Sometimes, you need to just format a single number, like an integer or long. In this case, you don't need to use string.Format. You can just use the ToString virtual method.

using System;

class Program
{
    static void Main()
    {
        int value = 123;
        string a = string.Format("{0:0000}", value); // Too complex.
        string b = value.ToString("0000"); // Simpler.
        Console.WriteLine(a);
        Console.WriteLine(b);
    }
}
0123
0123

Console

Programs can use format strings directly inside the Console.Write and Console.WriteLine methods. We can call Console.WriteLine with the same arguments as string.Format.

using System;

class Program
{
    static void Main()
    {
        string left = "hello";
        string right = "friend";
        Console.WriteLine("{0}, {1}!", left, right);
    }
}
hello, friend!

Hex is a number representation

We use format strings and the ToString method to convert ints to hex format. The int.Parse method can then be used to convert them back.

using System;
using System.Globalization;

class Program
{
    static void Main()
    {
        int value1 = 10995;

        // Write number in hex format.
        Console.WriteLine("{0:x}", value1);
        Console.WriteLine("{0:x8}", value1);

        Console.WriteLine("{0:X}", value1);
        Console.WriteLine("{0:X8}", value1);

        // Convert to hex.
        string hex = value1.ToString("X8");

        // Convert from hex to integer.
        int value2 = int.Parse(hex, NumberStyles.AllowHexSpecifier);
        Console.WriteLine(value1 == value2);
    }
}
2af3
00002af3
2AF3
00002AF3
True

FormatException

How can we solve this problem? System.FormatException is thrown by methods that receive format strings and substitutions.

{0}
{1}
{2}

FormatException, continued

Format strings use substitution markers. The arguments after the format string are placed in those markers in the same order.

using System;

class Program
{
    static void Main()
    {
        Console.WriteLine("{0} {2}", "x");
    }
}
Unhandled Exception: System.FormatException:
    Index (zero based) must be greater than or equal to zero and less
    than the size of the argument list.

String interpolation

For simple format strings, we can use string interpolation instead of the string.Format method. This has clearer syntax that is validated by the compiler.

using System;

class Program
{
    static void Main()
    {
        int value = 100;
        // String interpolation can be used instead of a format string.
        Console.WriteLine($"The size is {value}.");
    }
}
The size is 100.

StringBuilder

This class has a method called AppendFormat. The AppendFormat method receives a formatting string and the arguments to the formatting string.

using System;
using System.Text;

class Program
{
    static void Main()
    {
        var builder = new StringBuilder();
        // Use AppendFormat on StringBuilder.
        builder.AppendFormat("{0} and {1}", "cat", "dog");
        Console.WriteLine(builder);
    }
}
cat and dog

DateTime

When developing C# programs, we often need to place dates and times in strings. We can use format strings for this task.

A summary

With Format, we insert argument strings and separators together. We can specify display options for each argument. We combined strings and dates and numbers.