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

备注

The readonly keyword is different from the  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    */}