What is the difference between const and readonly in C#?

Below is a concise overview of how const and readonly differ in C#, along with tips on choosing the right one for different scenarios.

const (Compile-Time Constant)

• Must Be Known at Compile-Time
  A const value must be assigned at declaration. Once set, it cannot change during the execution of your program, and the compiler replaces all references with the literal value at compile time.

• Restricted Data Types
  It works best with primitive data types (e.g., int, float, bool, char, etc.) and string.

  public const int MaxItems = 100;

• Inlined by the Compiler
  Each usage of a const is compiled directly into the calling code. Changing a const in a library requires recompilation of all assemblies referencing it.

readonly (Run-Time Constant)

• Assigned at Run-Time
  A readonly field can be assigned either at declaration or within a constructor, allowing dynamic or runtime-defined values.

  public readonly int RunTimeValue;
  
  public SampleClass(int value)
  {
      RunTimeValue = value;
  }

• Not Inlined
  The compiler does not inline readonly values. If you change a readonly field in a library, only that library needs recompiling.

• Suitable for Reference Types
  readonly can be used for objects and arrays, making it possible to have a reference that can’t be changed once it’s been assigned—but the data within the object itself could still be mutable unless you make it immutable.

Choosing Between const and readonly

1. Immutability vs. Flexibility

   • Use const when the value is absolutely fixed and known at compile time.
   • Use readonly if you need to set the value at runtime, or if it’s not known until you create an instance.

2. Versioning Implications

   • Updating a const in a shared library means consumers must recompile their code.
   • A readonly field does not cause this issue—only the library defining it needs recompilation.

3. Complex/Reference Types

   • const is limited to simple, compile-time constant data types.
   • readonly can be applied to reference types, arrays, or other objects you want to assign once in the constructor.

Strengthening Your Coding Proficiency

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• Grokking the Coding Interview: Patterns for Coding Questions
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