c# - 当两个相同类型的对象具有相同的值时，为什么哈希码不同？

我的理解是 GetHashCode 将为共享相同值的两个不同实例返回相同的值。 MSDN 文档在这一点上有点模糊。



如果我有两个相同类型和相同值的实例，GetHashCode() 会返回相同的值吗？

假设所有值都相同，以下测试会通过还是失败？

SecurityUser 只有 getter 和 setter；

    [TestMethod]
    public void GetHashCode_Equal_Test()
    {
        SecurityUser objA = new SecurityUser(EmployeeName, EmployeeNumber, LastLogOnDate, Status, UserName);
        SecurityUser objB = new SecurityUser(EmployeeName, EmployeeNumber, LastLogOnDate, Status, UserName);

        int hashcodeA = objA.GetHashCode();
        int hashcodeB = objB.GetHashCode();

        Assert.AreEqual<int>(hashcodeA, hashcodeB);
    }


/// <summary>
/// This class represents a SecurityUser entity in AppSecurity.
/// </summary>
public sealed class SecurityUser
{
    #region [Constructor]

    /// <summary>
    /// Initializes a new instance of the <see cref="SecurityUser"/> class using the
    /// parameters passed.
    /// </summary>
    /// <param name="employeeName">The employee name to initialize with.</param>
    /// <param name="employeeNumber">The employee id number to initialize with.</param>
    /// <param name="lastLogOnDate">The last logon date to initialize with.</param>
    /// <param name="status">The <see cref="SecurityStatus"/> to initialize with.</param>
    /// <param name="userName">The userName to initialize with.</param>
    public SecurityUser(
        string employeeName,
        int employeeNumber,
        DateTime? lastLogOnDate,
        SecurityStatus status,
        string userName)
    {
        if (employeeName == null)
            throw new ArgumentNullException("employeeName");

        if (userName == null)
            throw new ArgumentNullException("userName");

        this.EmployeeName = employeeName;
        this.EmployeeNumber = employeeNumber;
        this.LastLogOnDate = lastLogOnDate;
        this.Status = status;
        this.UserName = userName;
    }

    #endregion

    #region [Properties]

    /// <summary>
    /// Gets the employee name of the current instance.
    /// </summary>
    public string EmployeeName { get; private set; }

    /// <summary>
    /// Gets the employee id number of the current instance.
    /// </summary>
    public int EmployeeNumber { get; private set; }

    /// <summary>
    /// Gets the last logon date of the current instance.
    /// </summary>
    public DateTime? LastLogOnDate { get; private set; }

    /// <summary>
    /// Gets the userName of the current instance.
    /// </summary>
    public string UserName { get; private set; }

    /// <summary>
    /// Gets the <see cref="SecurityStatus"/> of the current instance.
    /// </summary>
    public SecurityStatus Status { get; private set; }

    #endregion
}

最佳答案

框架为您的自定义对象计算的哈希码不能保证完全相同。

我相信这是由于框架没有遍历所有字段等并计算它们的哈希码，为每个对象做这件事会非常耗时(我可能是错的)。

这就是为什么建议您在自己的类型上覆盖 Equals() 和 GetHashCode() 方法的原因。

参见:Overriding GetHashCode