如何缓存具有多个属性的对象

Question

I'm looking to cache objects whose uniqueness is determined by a combination of all properties within that object. The object I have is something like this:

    public double A { get; set; }
    public double B { get; set; }
    public short C { get; set; }
    public bool D { get; set; }
    public double E { get; set; }
    public double F { get; set; }
    public double G { get; set; }
    public double H { get; set; }
    public double J { get; set; }
    public double K { get; set; }
    public double[] L { get; set; }
    public double[] M { get; set; }

I could overwrite GetHashCode and do something like return A ^ B ^ C etc... However, I'm concerned that I will have many collisions.

What would be the best way to cache an object such as this?

Answer 1

You can use this GetHashCode :

public override int GetHashCode()
{
    int hash = 23;
    unchecked
    {
        hash *= 17 + A.GetHashCode();
        hash *= 17 + B.GetHashCode();
        hash *= 17 + C.GetHashCode();
        // the same applies with the rest of your properties ...
        // collections must be treated differently:
        if(L != null)
        {
            hash *= 17 + L.Length;
            foreach(var d in L)
                hash *= 17 + d.GetHashCode();
        }
        if (M != null)
        {
            hash *= 17 + M.Length;
            foreach (var d in M)
                hash *= 17 + d.GetHashCode();
        }         
    }
    return hash;
}

This generates different hashcodes when different properties have the same value. If i would omit the prime-multipliers it wouldn't make a difference if A==A or A==B . The prime numbers are used to reduce the possibility of false collisions.

It also takes the arrays and their values+order into account.

This is a "must-read" on this topic: E. Lippert, Guidelines and rules for GetHashCode

Answer 2

A simple (altough probably not optimal) solution could be:

1. Generate a string representation of your class. If you had only escalar properties you could do something like string.Format("{0}-{1}-{2}", A, B, C) ; since you have arrays, you better use a StringBuilder and compose the string within a loop.

2. Invoke GetHashCode on the generated string.