C# Cookbook

3.25. Initializing a Constant Field at Runtime

Problem

A field marked as const can be initialized only at compile time. You need to initialize a field at runtime to a valid value, not at compile time. This field must then act as if it were a constant field for the rest of the application’s life.

Solution

When declaring a constant value in your code, there are two choices. You can use a readonly field or a const field. Each has its own strengths and weaknesses. However, if you need to initialize a constant field at runtime, you should use a readonly field:

public class Foo
{
    public readonly int bar;

    public Foo( ) {}

    public Foo(int constInitValue)
    {
        bar = constInitValue;
    }

    // Rest of class...
}

This is not possible using a const field. A const field can be initialized only at compile time:

public class Foo
{
    public const int bar;        // This line causes a compile-time error

    public Foo( ) {}

    public Foo(int constInitValue)
    {
        bar = constInitValue;    // This line also causes a compile-time error
    }

    // Rest of class...
}

Discussion

A readonly field allows initialization to take place only in the constructor at runtime, whereas a const field must be initialized at compile time. Therefore, implementing a readonly field is the only way to allow a field that must be constant to be initialized at runtime.

There are only two ways to initialize a readonly field. The first is by adding an initializer to the field itself:

public readonly int bar = 100;

The second way is to initialize the readonly field through ...