What is the equivalent of Java's final in C#?

 

Java在C＃中的final是什么？

The final keyword has several usages in Java. It corresponds to both the sealed and readonly keywords in C#, depending on the context in which it is used.

Classes

To prevent subclassing (inheritance from the defined class):

Java 

public final class MyFinalClass {...}

C# 

public sealed class MyFinalClass {...}

Methods

Prevent overriding of a virtual method.

Java 

public class MyClass
{
    public final void myFinalMethod() {...}
}

C# 

public class MyClass : MyBaseClass
{
    public sealed override void MyFinalMethod() {...}
}

As Joachim Sauer points out, a notable difference between the two languages here is that Java by default marks all non-static methods as virtual , whereas C# marks them as sealed . Hence, you only need to use the sealed keyword in C# if you want to stop further overriding of a method that has been explicitly marked virtual in the base class.

Variables

To only allow a variable to be assigned once:

Java 

public final double pi = 3.14; // essentially a constant

C# 

public readonly double pi = 3.14; // essentially a constant

As a side note, the effect of the readonly keyword differs from that of the const keyword in that the readonly expression is evaluated at runtime rather than compile-time, hence allowing arbitrary expressions.

It depends on the context.

  • For a final class or method, the C# equivalent is sealed .
  • For a final field, the C# equivalent is readonly .
  • For a final local variable or method parameter, there's no direct C# equivalent.

    • What everyone here is missing is Java's guarantee of definite assignment for final member variables.

    For a class C with final member variable V, every possible execution path through every constructor of C must assign V exactly once - failing to assign V or assigning V two or more times will result in an error.

    C#'s readonly keyword has no such guarantee - the compiler is more than happy to leave readonly members unassigned or allow you to assign them multiple times within a constructor.

    So, final and readonly (at least with respect to member variables) are definitely not equivalent - final is much more strict.

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