0.3.GetHashCode

遵守的原则:

1. equals相等则hashcode必须相等
2. hashcode相等equals不一定相等
3. 哈希表的key尽量使用简单类型,否则的话怎么构造一个有效的hash函数会是一个大问题

其他自定义的类一般怎么重写virtual gethashcode

1. return string1.GetHashCode()^string2.GetHashCode()
2. 这种方式非常奇怪,因为这样会导致同一类型的hashcode都相同
	也许它压根就不想成为
	type=GetType(); return type.GetHashCode();
	本质上是调用了Type类中gethashcode,本质上type又是调用的实现类RuntimeType
3. 也许默认直接调用父类Object的就可以了

复杂的是Single,Double,Decimal,String这些

0. Object
	return RuntimeHelpers.GetHashCode(this);
	public static extern int GetHashCode(Object o);		
1. Single
	float f = m_value;
    if (f == 0)
    {
        // Ensure that 0 and -0 have the same hash code
        return 0;
    }
    int v = *(int*)(&f);
    return v;
2. Double
	double d = m_value;
    if (d == 0)
    {
        // Ensure that 0 and -0 have the same hash code
        return 0;
    }
    long value = *(long*)(&d);
    return unchecked((int)value) ^ ((int)(value >> 32));
3. Decimal
	public extern override int GetHashCode();
4. String
	太复杂

基元类型中一般返回值,有三个修改了gethashcode

1. sbyte
	public override int GetHashCode() {
        return ((int)m_value ^ (int)m_value << 8);
    }
2. short 和char是一样的
	public override int GetHashCode() {
		return ((int)((ushort)m_value) | (((int)m_value) << 16));
    }
3. char 和short一样
	public override int GetHashCode()
    {
        return (int)m_value | ((int)m_value << 16);
    }

测试代码

List<ListValue> list = new List<ListValue>(13);

list.Add(new ListValue { type = "byte", value = (byte)1 });
list.Add(new ListValue { type = "sbyte", value = (sbyte)2 });
list.Add(new ListValue { type = "ushort", value = (ushort)3 });
list.Add(new ListValue { type = "short", value = (short)4 });
list.Add(new ListValue { type = "char", value = (char)'a' });
list.Add(new ListValue { type = "Int32", value = (Int32)6 });
list.Add(new ListValue { type = "UInt32", value = (UInt32)7 });
list.Add(new ListValue { type = "Int64", value = (Int64)8 });
list.Add(new ListValue { type = "UInt64", value = (UInt64)9 });
list.Add(new ListValue { type = "Single", value = (Single)10 });
list.Add(new ListValue { type = "Double", value = (Double)11 });
list.Add(new ListValue { type = "Decimal", value = (Decimal)12 });
list.Add(new ListValue { type = "string", value = "abcdef" });


foreach (ListValue item in list)
{
    Console.WriteLine($"{item.type}-{item.value}-{item.value.GetHashCode()}");
}

结果

byte-1-1
sbyte-2-514         //修改
ushort-3-3
short-4-262148		//修改
char-a-6357089	    //修改
Int32-6-6
UInt32-7-7
Int64-8-8
UInt64-9-9
Single-10-1092616192  
Double-11-1076232192
Decimal-12-1076363264
abcdef-abcdef-392335469