LinQ distinct with custom comparer leaves duplicates

Question

I\'ve got the following classes:

public class SupplierCategory : IEquatable
{
    public string Name { get; set; }
    public string          


        

Answer 1

Your problem is that you didn't implement IEqualityComparer correctly.

When you implement IEqualityComparer<T>, you must implement GetHashCode so that any two equal objects have the same hashcode.

Otherwise, you will get incorrect behavior, as you're seeing here.

You should implement GetHashCode as follows: (courtesy of this answer)

public int GetHashCode(List<SupplierCategory> obj) {
    int hash = 17;

    foreach(var value in obj)
        hash = hash * 23 + obj.GetHashCode();

    return hash;
}

You also need to override GetHashCode in SupplierCategory to be consistent. For example:

public override int GetHashCode() {
    int hash = 17;
    hash = hash * 23 + Name.GetHashCode();
    hash = hash * 23 + Parent.GetHashCode();
    return hash;
}

Finally, although you don't need to, you should probably override Equals in SupplierCategory and make it call the Equals method you implemented for IEquatable.

Answer 2

Actually, this issue is even covered in documentation: http://msdn.microsoft.com/en-us/library/bb338049.aspx.