Question

So, I need to create a struct in C# that will act as a key into a (quite large) dictionary, will look like this:

private readonly IDictionary<KeyStruct, string> m_Invitations;

Problem is, I REALLY need a struct to use as a key, because it is only possible to identify entries via two separate data items, where one of them can be a null (not only empty!) string.

What will I need to implement on the struct? How would you go about creating the hash? Would a hash collision (occassional) hurt the performance heavily or would that be negligible?

I’m asking because this is “inner loop” code.

Answer 1

If you have resharper, you can generate these method with Alt-Ins -> Equality members.

Here is the generated code for you KeyStruct:

public struct KeyStruct : IEquatable<KeyStruct>
{
    public string Value1 { get; private set; }
    public long Value2 { get; private set; }

    public KeyStruct(string value1, long value2)
        : this()
    {
        Value1 = value1;
        Value2 = value2;
    }

    public bool Equals(KeyStruct other)
    {
        return Equals(other.Value1, Value1) && other.Value2 == Value2;
    }

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(null, obj)) return false;
        if (obj.GetType() != typeof (KeyStruct)) return false;
        return Equals((KeyStruct) obj);
    }

    public override int GetHashCode()
    {
        unchecked
        {
            return ((Value1 != null ? Value1.GetHashCode() : 0)*397) ^ Value2.GetHashCode();
        }
    }

    public static bool operator ==(KeyStruct left, KeyStruct right)
    {
        return left.Equals(right);
    }

    public static bool operator !=(KeyStruct left, KeyStruct right)
    {
        return !left.Equals(right);
    }
}