class ClassidIntanceid
{
    public string i_classid { get; set; }
    public string ui_id { get; set; }
    public string i_instanceid { get; set; }
    public string i_market_hash_name { get; set; }
    public float ui_price { get; set; }
}

List<ClassidIntanceid> classintance = new List<ClassidIntanceid>();

classintance.GroupBy(o => (o.i_classid, o.i_instanceid)).Select(g => g.First())

classintance.GroupBy(o => (o.i_classid, o.i_instanceid), (key, g) => g.First())

class InstanceidComparer : EqualityComparer<ClassidIntanceid>
{
    public override bool Equals(ClassidIntanceid x, ClassidIntanceid y)
    {
        return x.i_classid == y.i_classid && x.i_instanceid == y.i_instanceid;
    }

    public override int GetHashCode(ClassidIntanceid obj)
    {
        return obj.i_classid.GetHashCode() * 17 + obj.i_instanceid.GetHashCode();
    }
}

var comparer = new InstanceidComparer();
var result = classintance.Distinct(comparer);