C# GetHashCode 的实现方式

这里给出一种普遍的做法：


版本号1：
实现一个helper。传递类型T。返回这个类型的hashcode。函数逻辑非常直接，仅仅是做了null check而已。假设obj不为空，则直接使用obj的hash code。

public class HashHelper
{
	private int _seed = 17;	
	public int Hash<T>(T obj)
	{
		// why 31?

		// https://computinglife.wordpress.com/2008/11/20/why-do-hash-functions-use-prime-numbers/
		// shortly, to reduce the conflict of hashing key‘s distrabution
		return 31 * _seed + ((obj == null) ?
 -1 : obj.GetHashCode());
	}
}

public class HashFluent
{
	private int _seed = 17;	
	private int _hashContext;
	
	public HashFluent Hash<T>(T obj)
	{
		// why 31?
		// https://computinglife.wordpress.com/2008/11/20/why-do-hash-functions-use-prime-numbers/
		// shortly, to reduce the conflict of hashing key‘s distrabution
		_hashContext = 31 * _seed + ((obj == null) ? -1 : obj.GetHashCode());
		return this;
	}
	
	public HashFluent Hash(int? value)
	{
		_hashContext = 31 * _seed + ((value == null) ? -1 : value.GetHashCode());
		return this;
	}
	
	public HashFluent Hash(IEnumerable sequence)
	{
		if (sequence == null)
		{
			_hashContext = 31 * _hashContext + -1;
		}
		else
		{
			foreach (var element in sequence)
			{
				_hashContext = 31 * _hashContext + ((element == null) ?
 -1 : element.GetHashCode());
			}
		}
		return this;
	}
	
	public override int GetHashCode (){
		return _hashContext;
	}
		
	// add more overridings here ..
	// add value types overridings to avoid boxing which is important
}