Is it possible to implement a immutable struct in c#

You are correct in observing that the readonly struct does not prevent reassignment. This is by design:

 

• readonly struct only ensures that the fields of the struct cannot be directly modified.
• It does not make the variable (the reference to the struct) immutable.

       

readonly struct Point
{
    public readonly int X { get; }
    public readonly int Y { get; }

    public Point(int x, int y)
    {
        X = x;
        Y = y;
    }
}

        In above defined structure, if you create references as 

           Point p1 = new Point(1, 2);

           Point p2 = new Point(3, 4);

           Then

      p1.X = 100; // Invalid

            p1 = p2; // Valid. Entirely replaces the instance, not its fields.

To create true immutability, you can try:

1. A readonly struct.
2. A readonly field or property to hold the struct ifield n  a class.

So, the implementation should be different using a class and your program only use that class to acess the structure.

class ImmutableStructHolder
{
    public readonly Point PointValue;

    public ImmutableStructHolder(Point point)
    {
        PointValue = point;
    }
}

 

Then 

var holder = new ImmutableStructHolder(new Point(1, 2)); //Valid

holder.PointValue = new Point(3, 4); // Error: Cannot assign to readonly field.