readonly和const区别

https://docs.microsoft.com/zh-cn/dotnet/csharp/language-reference/keywords/readonly

备注

The readonly keyword is different from the const keyword. A const field can only be initialized at the declaration of the field.A readonly field can be initialized either at the declaration or in a constructor. Therefore, readonly fields can have different values depending on the constructor used. Also, while a const field is a compile-time constant, the readonly field can be used for runtime constants as in the following example:

class Age
{
    readonly int year;
    Age(int year)
    {
        this.year = year;
    }
    void ChangeYear()
    {
        //year = 1967; // Compile error if uncommented.
    }
}

　　

class SampleClass
{
    public int x;
    // Initialize a readonly field
    public readonly int y = 25;
    public readonly int z;

    public SampleClass()
    {
        // Initialize a readonly instance field
        z = 24;
    }

    public SampleClass(int p1, int p2, int p3)
    {
        x = p1;
        y = p2;
        z = p3;
    }

    static void Main()
    {
        SampleClass p1 = new SampleClass(11, 21, 32);   // OK
        Console.WriteLine($"p1: x={p1.x}, y={p1.y}, z={p1.z}");
        SampleClass p2 = new SampleClass();
        p2.x = 55;   // OK
        Console.WriteLine($"p2: x={p2.x}, y={p2.y}, z={p2.z}");
    }
    /*
     Output:
        p1: x=11, y=21, z=32
        p2: x=55, y=25, z=24
    */
}