CODEWITHSUNDEEP
c#
Jonathan Kittell
Jonathan Kittell

Reputation: 7493

Why do two new objects not have the same hash code?

I wrote a custom comparer class.

public class ItemComparer : IEqualityComparer<Item>
{
    public int GetHashCode(Item x)
    {
        return (x == null) ? 0 : new { x.Name, x.CompanyCode,
           x.ShipToDate, x.Address }.GetHashCode();
    }

I have a test that fails when I new up two items and compare the hash codes. Why are the hashes different?

[TestMethod]
public void Two_New_Items_Have_The_Same_Hash_Code()
{
     // arrange
     var comparer = new ItemComparer();
     Item x = new Item();
     Item y = new Item();

     // act
     int xHash = comparer.GetHashCode(x);
     int yHash = comparer.GetHashCode(y);

     // assert
     Assert.AreEqual(xHash, yHash);
}

EDIT - Here is the complete class. I orginally used the example above for brevity but more information is needed

public class DtoPolicy : DtoBase
{
    [Description("The Policy Number")]
    public string PolicyNumber { get; set; }
    [Description("The Agent Code for the Agent who wrote the policy.")]
    public string AgentCode { get; set; }
    [Description("The First Name of the insured")]
    public string FirstName { get; set; }
    [Description("The Last Name of the insured")]
    public string LastName { get; set; }
    [Description("The Date of Birth of the insured")]
    public DateTime DateOfBirth { get; set; }
    [Description("The Age of the insured")]
    public int Age { get; set; }
    [Description("The Issue Date of the Policy")]
    public DateTime PolicyIssueDate { get; set; }
    [Description("The current status of the policy")]
    public string PolicyStatus { get; set; }
    public string TypeOfCoverage { get; set; }
    public string PlanDescription { get; set; }
    public decimal CompanyCode { get; set; }
    public DateTime? TerminationDate { get; set; }
    public decimal PolicyHolderSSN { get; set; }
    [Description("The Zip Code of the insured")]
    public string ZipCode { get; set; }
    public decimal OwnerSSN { get; set; }
    public string EmailAddress { get; set; }
    public string WebUsername { get; set; }
    public string OwnerFirstName { get; set; }
    public string OwnerLastName { get; set; }
    public string PayorFirstName { get; set; }
    public string PayorLastName { get; set; }
    public DateTime? PolicyEffectiveDate { get; set; }
    public string AgentName { get; set; }
    public string AgentPhone { get; set; }
    public string InsuredCityState { get; set; }
    public string InsuredAddress1 { get; set; }
    public string InsuredAddress2 { get; set; }
    public string InsuredCity { get; set; }
    public string InsuredState { get; set; }
    public string InsuredPhone { get; set; }
    public string OwnerAddress1 { get; set; }
    public string OwnerAddress2 { get; set; }
    public string OwnerCity { get; set; }
    public string OwnerState { get; set; }
    public string OwnerZip { get; set; }
    public string OwnerPhone { get; set; }
    public string PayorAddress1 { get; set; }
    public string PayorAddress2 { get; set; }
    public string PayorCity { get; set; }
    public string PayorState { get; set; }
    public string PayorZip { get; set; }
    public string PayorPhone { get; set; }
    public DateTime? PaidToDate { get; set; }
    public DateTime? LastPaidDate { get; set; }
    public string PremiumMode { get; set; }
    public decimal PremiumAmount { get; set; }
    public DateTime? LastBillDate { get; set; }
    public string BillingStatus { get; set; }
    public decimal TotalLoanAmount { get; set; }
    public decimal DividendAccumulation { get; set; }
    public decimal ModalPremiumMonthly { get; set; }
    public decimal ModalPremiumSemiAnnual { get; set; }
    public decimal ModalPremiumQuarterly { get; set; }
    public decimal ModalPremiumAnnual { get; set; }
    public bool ElectronicBilling { get; set; }
    public List<DtoClaim> Claims { get; set; }
    public decimal MarketCode { get; set; }
    public string BillingMode { get; set; }

    public DtoPolicy()
    {
        Claims = new List<DtoClaim>();
    }
}

This implementation of GetHashCode also returns different hashes for two new objects

public int GetHashCode(DtoPolicy x)
{
    return (x == null) ? 0 : x.Age.GetHashCode() ^ x.AgentCode.GetHashCode() ^ x.AgentName.GetHashCode() ^ x.AgentPhone.GetHashCode() ^
        x.BillingMode.GetHashCode() ^ x.BillingStatus.GetHashCode() ^
        x.Claims.GetHashCode() ^ x.CompanyCode.GetHashCode() ^ x.DateOfBirth.GetHashCode() ^ x.DividendAccumulation.GetHashCode() ^
        x.ElectronicBilling.GetHashCode() ^ x.EmailAddress.GetHashCode() ^ x.FirstName.GetHashCode() ^ x.InsuredAddress1.GetHashCode() ^
        x.InsuredAddress2.GetHashCode() ^ x.InsuredCity.GetHashCode() ^ x.InsuredCityState.GetHashCode() ^ x.InsuredPhone.GetHashCode() ^
        x.InsuredState.GetHashCode() ^ x.LastBillDate.GetHashCode() ^ x.LastName.GetHashCode() ^
        x.LastPaidDate.GetHashCode() ^ x.MarketCode.GetHashCode() ^ x.ModalPremiumAnnual.GetHashCode() ^ x.ModalPremiumMonthly.GetHashCode() ^
        x.ModalPremiumQuarterly.GetHashCode() ^ x.ModalPremiumSemiAnnual.GetHashCode() ^ x.OwnerAddress1.GetHashCode() ^ 
        x.OwnerAddress2.GetHashCode() ^ x.OwnerCity.GetHashCode() ^ x.OwnerFirstName.GetHashCode() ^
        x.OwnerLastName.GetHashCode() ^ x.OwnerPhone.GetHashCode() ^ x.OwnerSSN.GetHashCode() ^ x.OwnerState.GetHashCode() ^ 
        x.OwnerZip.GetHashCode() ^ x.PaidToDate.GetHashCode() ^ x.PayorAddress1.GetHashCode() ^
        x.PayorAddress2.GetHashCode() ^ x.PayorCity.GetHashCode() ^ x.PayorFirstName.GetHashCode() ^ x.PayorLastName.GetHashCode() ^
        x.PayorPhone.GetHashCode() ^ x.PayorState.GetHashCode() ^ x.PayorZip.GetHashCode() ^
        x.PlanDescription.GetHashCode() ^ x.PolicyEffectiveDate.GetHashCode() ^ x.PolicyHolderSSN.GetHashCode() ^ 
        x.PolicyIssueDate.GetHashCode() ^ x.PolicyNumber.GetHashCode() ^ x.PolicyStatus.GetHashCode() ^
        x.PremiumAmount.GetHashCode() ^ x.PremiumMode.GetHashCode() ^ x.TerminationDate.GetHashCode() ^
        x.TotalLoanAmount.GetHashCode() ^ x.TypeOfCoverage.GetHashCode() ^ x.WebUsername.GetHashCode() ^ x.ZipCode.GetHashCode();
}

Upvotes: 6

Views: 182

Answers (2)

Rob
Rob

Reputation: 4477

If I define the Item class as

public class Item
{
    public string Name { get; set; }
    public string CompanyCode { get; set; }
    public DateTime ShipToDate { get; set; }
    public List<string> Address { get; set; }

    public Item()
    {
        //Uncomment Address initialization and the test fails..
        //Address = new List<string>(); 
    }
}

Then your test passes for me. What are the types of these properties, and how are they initialized in the parameterless constructor?

Edit: Following your update - it's presumably the new List<DtoClaim>() created in the DtoPolicy constructor.

Upvotes: 3

Boas Enkler
Boas Enkler

Reputation: 12557

I assume that you have complex types in one of your proeprties. Therefore: When having complex type the GetHashCode is dirffent for each instanciated object.

You may have to implement a more detailed gethashcode method.,

I assume you want to join the hashcodes of each property for example like this(untested) 

x.Name.GetHashCode() ^ x.CompanyCode.GetHashCode() ^ x.ShipToDate.GetHashCode() ^  x.Address.Id

This post on how to implement GetHashCode for structure describes the hashcode implementation for multiple properties (in this case with a struct).

Upvotes: 6

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