可以在构造函数中初始化但之后只读的 C# 抽象属性？

Question

In a class I have a property

protected abstract string test{ get; }

When I try to initialize it in a constructor. I get an error saying:

Property or indexer xxx cannot be assigned to. -- it is read-only.

Is there a way to allow some property to be

1. abstract
2. Read-only after initialization in ctor ?

Answer 1

You likely don't want an abstract property. You would only use that if you wanted to force the derived class to provide a custom implementation. In your case you simply want it to be set in the constructor and readonly .

public abstract class Base
{
    protected string MyProperty { get; }

    public Base(string myProperty)
    {
        MyProperty = myProperty;
    }
}

public class Derived : Base
{
    public Derived()
        : base("DefaultValue")
    { }
}

Answer 2

Simply implement the abstract method with a private setter in your derived class:

class Derived : Base
{
    protected override string test { get; } = "MyInitialValue";
}

or before C#6 with a readonly backing-field:

class Derived : Base
{
    private readonly string _test = "MyInitialValue";
    protected override test { get { return this._test; } }
}

This allows you to set different values for different classes. If you don´t need this and all classes should have the exact same value you can define it within your base-class my making it non-abstract.

You can also use a private setter in your derived class. This makes it readonly to other classes but you can set the value within your class outside the constructor also.