Does C# Distinct() method keep original ordering of sequence intact?

Question

I want to remove duplicates from list, without changing order of unique elements in the list.

Jon Skeet & others have suggested to use following

Answer 1

By default when use Distinct linq operator uses Equals method but you can use your own IEqualityComparer object to specify when two objects are equals with a custom logic implementing GetHashCode and Equals method. Remember that:

GetHashCode should not used heavy cpu comparision ( eg. use only some obvious basic checks ) and its used as first to state if two object are surely different ( if different hash code are returned ) or potentially the same ( same hash code ). In this latest case when two object have the same hashcode the framework will step to check using the Equals method as a final decision about equality of given objects.

After you have MyType and a MyTypeEqualityComparer classes follow code not ensure the sequence maintain its order:

var cmp = new MyTypeEqualityComparer();
var lst = new List();
// add some to lst
var q = lst.Distinct(cmp);

In follow sci library I implemented an extension method to ensure Vector3D set maintain the order when use a specific extension method DistinctKeepOrder:

relevant code follows:

/// 

/// support class for DistinctKeepOrder extension
/// 
public class Vector3DWithOrder
{
    public int Order { get; private set; }
    public Vector3D Vector { get; private set; }
    public Vector3DWithOrder(Vector3D v, int order)
    {
        Vector = v;
        Order = order;
    }
}

public class Vector3DWithOrderEqualityComparer : IEqualityComparer
{
    Vector3DEqualityComparer cmp;

    public Vector3DWithOrderEqualityComparer(Vector3DEqualityComparer _cmp)
    {
        cmp = _cmp;
    }

    public bool Equals(Vector3DWithOrder x, Vector3DWithOrder y)
    {
        return cmp.Equals(x.Vector, y.Vector);
    }

    public int GetHashCode(Vector3DWithOrder obj)
    {
        return cmp.GetHashCode(obj.Vector);
    }
}

In short Vector3DWithOrder encapsulate the type and an order integer, while Vector3DWithOrderEqualityComparer encapsulates original type comparer.

and this is the method helper to ensure order maintained

/// 

/// retrieve distinct of given vector set ensuring to maintain given order
/// 
        
public static IEnumerable DistinctKeepOrder(this IEnumerable vectors, Vector3DEqualityComparer cmp)
{
    var ocmp = new Vector3DWithOrderEqualityComparer(cmp);

    return vectors
        .Select((w, i) => new Vector3DWithOrder(w, i))
        .Distinct(ocmp)
        .OrderBy(w => w.Order)
        .Select(w => w.Vector);
}

Note : further research could allow to find a more general ( uses of interfaces ) and optimized way ( without encapsulate the object ).