KeyNotFoundException：字典中不存在给定的键

Question

I am receiving KeyNotFoundException whenever I try to access the key in dictionary.

I have a PricingRules class which has a dictionary:

public class PricingRules
{
    Dictionary<ItemCode, Money> pricing_rules;

    public PricingRules()
    {
        pricing_rules = new Dictionary<ItemCode, Money>();
        pricing_rules.Add(new ItemCode("A"), new Money(50));
        pricing_rules.Add(new ItemCode("B"), new Money(30));
        pricing_rules.Add(new ItemCode("C"), new Money(20));
        pricing_rules.Add(new ItemCode("D"), new Money(15));
    }

    public void itemScanned(ItemCode itemCode, Money amount)
    {
        amount.Add(pricing_rules[itemCode]);
    }

    public Money AmountForItem(ItemCode itemCode)
    {
        return pricing_rules[itemCode];
    }
}

The key in the dictionary is of type ItemCode :

public class ItemCode
{
    private readonly string itemCode;

    public ItemCode(string itemCode)
    {
        this.itemCode = itemCode;
    }

    public override int GetHashCode()
    {
        return itemCode.GetHashCode();
    }
}

and the value is of type Money :

public class Money
{
    private int amount;

    public Money(int amount)
    {
        this.amount = amount;
    }

    public override bool Equals(object obj)
    {
        Money money = (Money)obj;
        return this.amount == money.amount;
    }

    public void Add(object obj)
    {
        Money money = (Money)obj;
        this.amount += money.amount;
    }

    public override int GetHashCode()
    {
        return amount.GetHashCode();
    }
}

I have tried overriding the GetHashCode() function to return the HashCode of the primitive type in the class but it still is giving KeyNotFoundException

The test I have written is:

public class PricingRulesShould
{
    [Fact]
    void ReturnValueFiftyWhenKeyIsA()
    {
        Money expected = new Money(50);
        PricingRules pr = new PricingRules();

        ItemCode itemcode = new ItemCode("A");
        Money actual = pr.AmountForItem(itemcode);

        Assert.Equal(expected, actual);
    }
}

Answer 1

When Dictionary accesses its elements, it looks for the equality of keys.
Since the ItemCode is not a primitive type, when you compare new ItemCode("A") == new ItemCode("A") you should get false as a result, because they are different instances with identical contents.
What you can do is to implement IEquatable<ItemCode> by the ItemCode class:

public class ItemCode : IEquatable<ItemCode>
{
    private readonly string itemCode;

    public ItemCode(string itemCode)
    {
        this.itemCode = itemCode;
    }

    public override int GetHashCode()
    {
        return itemCode != null ? itemCode.GetHashCode() : 0;
    }

    public bool Equals(ItemCode other)
    {
        if (ReferenceEquals(null, other)) return false;
        if (ReferenceEquals(this, other)) return true;
        return string.Equals(itemCode, other.itemCode);
    }

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(null, obj)) return false;
        if (ReferenceEquals(this, obj)) return true;
        return obj is ItemCode ic && Equals(ic);
    }
}