Entity Framework and T4: Generating GetHashCode() for your entities


A simple Entity Framework T4 template that generates GetHashCode() method

Table of contents

GetHashCode() overview

A hash code is a numeric value that is used to identify an object during equality testing, and it can also serve as an index for an object in a collection, such as a List, Dictionary or HashTable.

The problem is that the default .NET implementation of GetHashCode() does not guarantee unique values for different values, so you should override this method and provide your own implementation.

Some rules for implementing GetHashCode() are:

  • If two objects compare as equal, the GetHashCode method for each object must return the same value. However, if two objects do not compare as equal, the GetHashCode methods for the two object do not have to return different values.
  • The GetHashCode method for an object must consistently return the same hash code as long as there is no modification to the object state
  • For the best performance, a hash function must generate a random distribution for all input
  • Implementations of the GetHashCode method must not result in circular references (it can lead to a StackOverflowException).
  • Implementations of the GetHashCode method must not throw exceptions.

This is a short overview of GetHashCode() method, taken from MSDN. You can read more here: Object.GetHashCode Method

Implementing GetHashCode()

I found a great post on StackOverflow on this topic. Jon Skeet has provided a good and simple implementation. Basically, you need to use prime numbers like 17, 23, 29, 31 in the hash code calculation. I decided to include also the type of the object in the calculation, because two objects with identical properties/values can return the same hash code.

public class Organisation
{
	public int Id { get; set; }
	public string Name { get; set; }
	public string Country { get; set; }

	public override int GetHashCode()
	{
		unchecked
		{
			int multiplier = 31;
			int hash = GetType().GetHashCode();

			hash = hash * multiplier + Id.GetHashCode();
			hash = hash * multiplier + (Name == null ? 0 : Name.GetHashCode());
			hash = hash * multiplier + (Country == null ? 0 : Country.GetHashCode());

			return hash;
		}
	}
}

Using T4 templates to generate GetHashCode()

T4 is a code generator built right into Visual Studio. You can generate any text file using T4 templates: C#, javascript, HTML, XML and many others. If you’ve never heard about it, this is a good place to start:

T4 (Text Template Transformation Toolkit) Code Generation – Best Kept Visual Studio Secret

I’ve modified the ADO.NET C# POCO Entity Generator template to generate GetHashCode() method for each entity in the model. Feel free to download the demo project (VS2010).

References

Downloads

Download the demo project (VS2010): T4-GetHashCode.rar

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4 thoughts on “Entity Framework and T4: Generating GetHashCode() for your entities

  1. Your initial hash should be a prime number,
    int hash = GetType().GetHashCode();
    should be
    int hash = 17;
    Makes it less likely to have collision issues.

    Also why would you use code generation over reflection in this case?

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