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C# string.Format Method

This C# tutorial uses string.Format to insert values into a pattern string. It uses percentages, decimals and padding.

Format. Variables contain values.

An int contains 100. A string contains a name. These values have no formatting information. We must format them with string.Format, a static method.

With Format, we apply formatting (and add surrounding text). We specify substitution markers. Many methods (string.Format, Console.WriteLine, ToString, AppendFormat) use format syntax.

First example. Here we use string.Format to combine three strings with formatting options. The format string itself is the first argument. It is usually specified as a string literal.

Markers: The 0, 1 and 2 are where the first, second and third arguments are inserted. A format specification comes after the ":" char.

Variables: The program formats three variables. These are a string, an int and a DateTime struct.

Result: The string has formatting applied. The int is displayed with a decimal place. The year is displayed with four digits.

Based on:

.NET 4.5

C# program that uses string.Format

using System;

class Program
    static void Main()
	// Declare three variables.
	// ... The values they have are not important.
	string value1 = "Dot Net Perls";
	int value2 = 10000;
	DateTime value3 = new DateTime(2015, 11, 1);
	// Use string.Format method with four arguments.
	// ... The first argument is the formatting string.
	// ... It specifies how the next arguments are formatted.
	string result = string.Format("{0}: {1:0.0} - {2:yyyy}",
	// Write the result.


Dot Net Perls: 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.

Note: The format string in this example uses the 0:0.0% syntax. This means that the second argument is formatted with the pattern 0.0%.

C# program that uses string.Format with number

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%}",


string = 73.0%

Numbers, percentages. The 0.0% part of the format string specifies the number of digits. The format code can have many digits before the decimal place, but only one after it.

The presence of a "%" character in a format string causes a number to be multiplied by 100 before it is formatted.

Custom Numeric Format Strings: MSDN

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.

Tip: Instead of the PadLeft and PadRight methods, you can use the string.Format method with special substitutions.

PadRight, PadLeft

Sizes: Use the comma char followed by the padding size. A negative number will add padding to the right (left-align).

Also: You can use a positive padding size to add padding to the left. This will right-align the string.

C# program that uses string.Format for padding

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,
	string line2 = string.Format(format,
	// Write the formatted strings.


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.


C# program that uses string.Format and ToString

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.



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.

Code: X formats as hexadecimal. You can specify X and then a number (such as X8) to pad the output on the left side with zeros.

Tip: You can use the NumberStyles.AllowHexSpecifier argument to make int.Parse handle hex numbers.

C# program that handles hex number conversions

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);



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

Substitution markers:


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

Here: The System.FormatException is thrown because the {2} substitution marker was not found in the argument list.

Tip: To fix the program, you could remove the substitution marker {2} or add two more arguments.

Tip 2: Whenever you encounter a FormatException, it is worthwhile to check substitution markers and argument lists for formatting methods.

C# program that throws an exception

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.

Dates. DateTime, a struct, represents any possible date and time. The string.Format method can be used with DateTime arguments. These will be inserted into the substitution markers.

DateTime, Format

File names. Many programs need to create files dynamically. The files often need to have file names that are based on some characteristic.

For example: A logging file can have a file name that is based on the exact date and time it was written.

Tip: You can use the string.Format method with substitutions to create the file names based on state variables.

Filename, DateTime

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


Console. Programs can use format strings directly inside the Console.Write and Console.WriteLine methods. In Visual Studio, type Console.WriteLine. Scroll through the IntelliSense window.


Then: You can call Console.WriteLine with the same arguments as the string.Format method receives. It will call string.Format.

Implementation. The string.Format method is implemented with a params argument—this is a variable parameter list. This results in an array allocation on each invocation of the method.

Params: We discover more about the params keyword here. A method with params creates an array on each call.


Substitution processing. Internally, string.Format uses StringBuilder, which contains a mutable character buffer. It estimates a capacity based on a heuristic.

StringBuilder Capacity

Next: The AppendFormat method is called to process the substitutions themselves.

Finally: The ToString method is called. It does not normally require a copy to be made.

StringBuilder ToString

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.

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