How to implement C# GetHashCode

In a project, when using a hash table, you sometimes need to Override GetHashCode. Here is a common approach:

Version 1:
Implement a helper, pass type T, and return the hashcode of this type. The function logic is very straightforward, it just does a null check; if obj is not empty, the hash code of obj is used directly.

public class HashHelper
{
	private int _seed = 17;	
	public int Hash<T>(T obj)
	{
		// why 31?
		// http://www.php.cn/
		// shortly, to reduce the conflict of hashing key&#39;s distrabution
		return 31 * _seed + ((obj == null) ? -1 : obj.GetHashCode());
	}
}

Why use magic number 31? Using prime number products can relatively increase uniqueness and reduce conflicts in hash key value allocation; and 31 is for Compiler optimization considerations (effectively converts to i<<5-1). After some searching, this implementation method comes from the hash code function of string in JAVA. Here is a detailed introduction:
https://computinglife.wordpress.com/2008/11/20/why-do-hash-functions-use-prime-numbers/
Implementation version 2:
can be extended This class becomes a fluent interface, which can hash various types. For value types, the significance of overloading is to reduce boxing; for collections or generics, it is to make external calls more natural and more readable.

public class HashFluent
{
	private int _seed = 17;	
	private int _hashContext;
	
	public HashFluent Hash<T>(T obj)
	{
		// why 31?
		// http://www.php.cn/
		// shortly, to reduce the conflict of hashing key&#39;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
}

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