C# How can I compare two lists of objects in the Equals()-Method

Question

I have two Lists of objects with the same elements and values:

parameters = new List<Parameter>() { new Parameter() { parameterName="value", parameterType="string"} }

the Parameter class looks like this:

 public class Parameter
    {
        public string parameterName;

        public string parameterType;
    }

and I want to compare it with an identical List of objects with the same Elements and Values, via an unit-test.

my Method Class(I created before Method objects, which have stored a List of Parameter objects):

public class Method
{
   public List<Parameter> parameters { get; set;}

   public string modifier { get; set; }

   public string name { get; set; }

   public string type { get; set; }


   public override bool Equals(object obj)
        {
            return this.modifier == ((Method)obj).modifier && this.name == ((Method)obj).name && this.type == ((Method)obj).type
                && this.parameters == ((Method)obj).parameters; 
        }
}

my problem is in the Equals method, at the point of this.parameters == ... :

public override bool Equals(object obj)
        {
            return this.modifier == ((Method)obj).modifier && this.name == ((Method)obj).name && this.type == ((Method)obj).type
                && this.parameters == ((Method)obj).parameters; 
        }

By the way, all other criteria in this method are working. Modifier, name and type are returning the right value compared to the object. Because of these are basic string values and the comparison is much easier. It gets more complicated when I want to do the same for the List of objects.

How can I compare two Lists of Parameter-objects in this Equals()-Method, do I need to compare the elements each(like comparing parameterName and parameterType of all objects in the list)?

Are you going to compare for equality all elements in List parameters? — Kamiky, Dec 27 '19 at 16:31
Does this answer your question? [How can I compare two lists with xunit test](https://stackoverflow.com/questions/59458136/how-can-i-compare-two-lists-with-xunit-test) — panoskarajohn, Dec 27 '19 at 17:04

score 4 · Answer 1 · edited Sep 12 '20 at 15:28

You need to override Equals in your Parameter class as well as overriding GetHashCode in order to maintain hashes (otherwise you will get different hashes for 2 objects that have same values).

public class Parameter
{
    public string parameterName;

    public string parameterType;

    public override bool Equals(object obj)
    {
        return parameterName == ((Parameter) obj)?.parameterName &&
               parameterType == ((Parameter) obj)?.parameterType;
    }

    public override int GetHashCode()
    {
        unchecked
        {
            var hashCode = parameterName != null ? parameterName.GetHashCode() : 0;
            hashCode = (hashCode * 397) ^ (parameterType != null ? parameterType.GetHashCode() : 0);
            return hashCode;
        }
    }
}

And then use SequenceEqual when comparing Lists:

public class YourClass
{
    public List<Parameter> parameters { get; set; }

    public string modifier { get; set; }

    public string name { get; set; }

    public string type { get; set; }
    
    public override bool Equals(object obj)
    {
        return modifier == ((YourClass)obj)?.modifier && 
               name == ((YourClass)obj)?.name && 
               type == ((YourClass)obj)?.type && 
               ( parameters == null && ((YourClass)obj)?.parameters == null || 
                 parameters != null && ((YourClass)obj)?.parameters != null && parameters.SequenceEqual(((YourClass)obj).parameters));
    }
    
    public override int GetHashCode()
    {
        unchecked
        {
            var hashCode = parameters != null ? parameters.GetHashCode() : 0;
            hashCode = (hashCode * 397) ^ (modifier != null ? modifier.GetHashCode() : 0);
            hashCode = (hashCode * 397) ^ (name != null ? name.GetHashCode() : 0);
            hashCode = (hashCode * 397) ^ (type != null ? type.GetHashCode() : 0);
            return hashCode;
        }
    }
}

Side note: check the types before trying to cast them out... as well as if they are `null`. — Trevor, Dec 27 '19 at 16:51
@Roman.Pavelko If you override `Equals` you must override `GetHashCode` as well such that "equal" objects return the same hash code. — D Stanley, Dec 27 '19 at 17:23
@Roman.Pavelko It looks like the hashCode would be overwritten on each line. Would you need a hashCode += on each line except the first ? — Rich, May 13 '20 at 16:59
@Rich actually it will not be overwritten thanks to ^ operator — Roman.Pavelko, May 13 '20 at 19:53
@Roman.Pavelko That's interesting. I haven't used that operator much but now after looking it up I see what it does. — Rich, May 13 '20 at 21:00

score 1 · Answer 2 · answered Dec 27 '19 at 16:35

Terminology: "Compare" and "Equal" are two different things, Compare means finding out an order (which one comes first), while Equals just tests yes/no equal or not (identity).

You have to define for yourself what equal means to you. Is {1;2;3} the same as {3;2,1;} ? Not only you have to define, what equal lists are, but also what equal parameters are.

For the Compare of list you could use

 parameters.SequenceEquals(((other)Method)).parameters);

if order is important to you.

To compare the Parameters itself, you have to override the Equals of the parameter, or provide a custom EqualityComparer that you can pass to SequenceEquals.

Which actually does nothing else than comparing one by one, and breaking on the first mismatch.

C# How can I compare two lists of objects in the Equals()-Method

2 Answers2