代码之家 › 专栏 › 技术社区 › cdmckay

我应该使用字符串字段的串联作为散列码吗?

iequatable equality hashcode string c#

cdmckay · 技术社区 · 15 年前

我在C#中有一个Address类,看起来像这样:

public class Address
{            
    public string StreetAddress { get; set; }
    public string RuralRoute { get; set; }
    public string City { get; set; }
    public string Province { get; set; }
    public string Country { get; set; }
    public string PostalCode { get; set; }
}

即:

var str = new StringBuilder();
str.Append(StreetAddress)
   .Append(RuralRoute)
   ...

return str.ToString().GetHashCode();

这有什么优点/缺点?为什么我不应该这样做?

4 回复 | 直到 15 年前

Community Maksym Gontar 7 年前

事实上

我会选择我之前提到的“简单且容易找到正确的”算法 used in this answer (感谢您查找lance:)-正如您所说,它在有效Java中列出。在这种情况下,其结果将是:

public int GetHashCode()
{
    int hash = 17;
    // Suitable nullity checks etc, of course :)
    hash = hash * 23 + StreetAddress.GetHashCode();
    hash = hash * 23 + RuralRoute.GetHashCode();
    hash = hash * 23 + City.GetHashCode();
    hash = hash * 23 + Province.GetHashCode();
    hash = hash * 23 + Country.GetHashCode();
    hash = hash * 23 + PostalCode.GetHashCode();
    return hash;
}

当然,这不是零安全的。如果您使用的是C 3,那么您可能需要考虑一种扩展方法:

public static int GetNullSafeHashCode<T>(this T value) where T : class
{
    return value == null ? 1 : value.GetHashCode();
}

public int GetHashCode()
{
    int hash = 17;
    // Suitable nullity checks etc, of course :)
    hash = hash * 23 + StreetAddress.GetNullSafeHashCode();
    hash = hash * 23 + RuralRoute.GetNullSafeHashCode();
    hash = hash * 23 + City.GetNullSafeHashCode();
    hash = hash * 23 + Province.GetNullSafeHashCode();
    hash = hash * 23 + Country.GetNullSafeHashCode();
    hash = hash * 23 + PostalCode.GetNullSafeHashCode();
    return hash;
}

你能够

public static int GetHashCode(params object[] values)
{
    int hash = 17;
    foreach (object value in values)
    {
        hash = hash * 23 + value.GetNullSafeHashCode();
    }
    return hash;
}

public int GetHashCode()
{
    return HashHelpers.GetHashCode(StreetAddress, RuralRoute, City,
                                   Province, Country, PostalCode);
}

在大多数类型中,都会涉及到原语,因此执行装箱操作有些不必要,但在这种情况下,您只能使用引用。当然,你最终会不必要地创建一个数组,但是你知道他们怎么说过早优化。。。

Kosi2801 15 年前

想想

"StreetAddress" + "RuralRoute" + "City"

"Street" + "AddressRural" + "RouteCity"

两者的哈希代码相同,但字段中的内容不同。

Keltex 15 年前

IEqualityComparer<Address> :

public class Address : IEqualityComparer<Address>
{        
    //
    // member declarations
    //

    bool IEqualityComparer<Address>.Equals(Address x, Address y)
    {
        // implementation here
    }

    int IEqualityComparer<Address>.GetHashCode(Item obj)
    {
        // implementation here
    }
}

您还可以实现 IComparable<Address> 要获得订单。。。

-4

Wouter Asim 9 年前

public string getfourDigitEncryptedText(string input) { 
    int hashCode = input.hashCode(); 
    string hstring = (new StringBuilder()).append(hashCode).append("").toString(); 
    string rev_hstring = (new StringBuilder(hstring)).reverse().toString(); 
    string parts[] = rev_hstring.trim().split(""); 
    int prefixint = 0; 
    for(int i = 1; i <= parts.length - 3; i++) 
        prefixint += integer.parseInt(parts[i]);   
    string prefixstr = "0"; 
    if((new integer(prefixint)).toString().length() < 2) 
        prefixstr = (new StringBuilder()).append((new integer(prefixint)).toString()).append("5").toString(); 
    else if((new integer(prefixint)).toString().length() > 2) 
        prefixstr = "79"; 
    else 
        prefixstr = (new integer(prefixint)).toString(); 
    string finalstr = (new StringBuilder()).append(prefixint).append(rev_hstring.substring(3, 5)).toString();
    return finalstr; 
}