Const

The C# const 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.

It is impossible to reassign a constant. With constants, we lose the ability to modify variables at runtime but we gain performance and compile-time validation.

Example

With const, we move constant values into a single part of the program. The values are inserted into the parts of the program that use them—there is no lookup overhead.

Part 1 Here we access the _html constant string in 2 ways. We can read a constant in the same way as a static field.

Part 2 A constant can be placed in the local scope. Other than scope, there is no difference between a local const and a global const.

Part 3 We cannot reassign consts in a program. This would result in a compile-time error.

using System;

class Program
{
    const string _html = ".html";

    static void Main()
    {
        // Part 1: access constant.
        Console.WriteLine(_html);
        Console.WriteLine(Program._html);

        // Part 2: local constant.
        const string txt = ".txt";
        Console.WriteLine(txt);

        // Part 3: invalid statements.
        // ... This causes a compile-time error.
        // _html = "";
        // ... This causes a compile-time error also.
        // txt = "";
    }
}.html
.html
.txt

Left-hand side error

This is what happens when we try to mutate a constant value. We get a C# compile-time error. This program won't go far in life.

class Program
{
    static void Main()
    {
        const int value = 10;
        value = 20;
    }
}Error CS0131
The left-hand side of an assignment must be a variable, property or indexer

Constant error

All parts of a constant expression must themselves be constant. So we cannot buildup a const from variables. No variables are allowed.

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

class Program
{
    static void Main()
    {
        int size = 5;
        const int value = 10 + size;
    }
}Error CS0133
The expression being assigned to 'value' must be constant

Const, notes

Constants promote the integrity of programs by telling the compiler not to allow changes to the values during runtime. This should result in fewer accidental bugs.

Also Const may improve performance over variables in many cases, particularly when using types like int.

Summary

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