What is the difference between const and readonly (C#) / final (Java)?
const, readonly and final
These keywords all relate to values that should not be changed after a particular point, but their semantics differ significantly. In C#, const represents a compile-time constant and is implicitly static; its value must be known at compile time. readonly is normally an instance or static field that can be assigned only at declaration or in a constructor. In Java, final prevents reassignment of a variable after it has been initialized, but for object references it does not make the referenced object immutable. Therefore, final is closer to 'cannot reassign this variable' than to C# const.
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C# const: compile-time constant; implicitly static.
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C# readonly: field can be assigned at declaration or in a constructor and then cannot be reassigned.
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Java final: variable/reference cannot be reassigned after initialization.
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A final or readonly reference does not automatically make the referenced object immutable.
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Immutable object design requires the object's own state to be protected from mutation.
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Compile-time constants can have binary compatibility implications when consumed across assemblies/modules, so public constants should be designed carefully.
Follow-up Questions
- Is readonly the same as immutable?
- Can a readonly field be assigned in a constructor?
- What does final mean for an object reference in Java?
- What is the difference between const and static readonly in C#?
- What are binary compatibility concerns with public constants?