C# 3.0 Cookbook, 3rd Edition

3.14. 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 to a valid value at runtime, 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

You have two choices when declaring a constant value in your code. 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 must 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 ...