C# Const Example

Const, a keyword, indicates a constant. It describes an entity that cannot be changed at program runtime. Instead, the entity must be fully resolved at compile-time. We cannot reassign a constant.

Example. Const is a reserved keyword. It allows us to pull constant values (such as strings or integral types) into a part of the program that is easier for us to manage and edit. This improves program organization.

Tip: When you compile const values, the values are inserted into the parts of the program that use them.

And: This eliminates any variable lookup overhead. Const declarations at the class level are retained in the assembly metadata.

Based on:

.NET 4.5

C# program that uses const

using System;

class Program
    const string _html = "";

    static void Main()
	// This causes a compile-time error:
	// _html = "";

	Console.WriteLine(_html);         // Access constant
	Console.WriteLine(Program._html); // Access constant

	const string txt = ".txt";

	// This causes a compile-time error also:
	// txt = "";




The program contains two assignment statements that are commented out with single-line comments. They reference const identifiers. They would result in a compile-time error if they were uncommented.

Note: Compile-time errors are separate from exceptions and are raised before you can ever execute the program.

You cannot assign const identifiers after compile-time. But you can access them as much as you want. The program above accesses the _html constant string in two ways. You can read a constant in the same way as a static variable.

Static Field

Static: You can also describe the constant with the public accessibility modifier to use it throughout your program.

Errors. When refactoring your program, you may run into some errors related to constant fields. The following examples shows the compile-time error that will occur if you try to assign constants at runtime after they are already assigned.

Tip: To fix this error, either modify the constant to make it a variable, or remove the assignment.

Compile-Time Error

Error: The left-hand side of an assignment must be a variable, property or indexer.

Advantages. Constant fields have advantages in C# programs. They promote the integrity of your programs by telling the compiler not to allow you to change the values during runtime, which can result in fewer accidental bugs.

They also improve performance over regular variables in many cases, particularly when using integral types such as int. They eliminate accesses to memory that occur with fields and locals.


Magic numbers are literal numbers, such as 100 or 47524, that appear in the middle of a program without explanation. If you program in a language that supports named constants, use them instead.

Code Complete

Const is a reserved word. It allows us to specify that a value is invariant and must not be modified after compile-time. Const values, like const strings, help us simplify and optimize programs.