C#

ErrorProvider errProvider;
List<ErrorProvider> errProviderLst = new List<ErrorProvider>();

...

errProvider = new ErrorProvider();
errProviderLst.Add(errProvider);
errProvider.SetError(c, "Ange ett heltal");

...

errProviderLst[i].Clear();
errProviderLst.RemoveAt[i];

using System;
using System;
using System.Collections.Generic;
using System.Runtime.Serialization;
using System.IO;

namespace WhatEver
{
    public interface ISomeInterface
    {
        DateTime Date { set; get; }
    }

    public class BunchOfISomeInterface : Dictionary<Type, List<ISomeInterface>>
    {
    }

    [DataContract]
    public classLite : ISomeInterface
    {
        [DataMember]
        public DateTime Date { set; get; }
    }

    [DataContract]
    public classRegular : ISomeInterface
    {
        [DataMember]
        public DateTime Date { set; get; }

        [DataMember]
        public extraString { set; get; }
    }

    public static class WriteMe
    {
        private static DataContractSerializer serializer;

        static string filepath = @"C:\test.xml";

        public static void Write(BunchOfISomeInterface bunch)
        {
            using (FileStream writer = new FileStream(filepath, FileMode.OpenOrCreate, FileAccess.Write))
            {
                serializer = new DataContractSerializer(typeof(BunchOfISomeInterface));

                // error here
                serializer.WriteObject(writer, bunch);
            }
        }
    }
}

using WhatEver;

private void SomeTestClass()
{
    BunchOfISomeInterface currentBunch = new BunchOfISomeInterface();
    
    Lite lite1 = new Lite();
    lite1.Date = DateTime.Now;

    Regular regular1 = new Regular();
    regular1.Date = DateTime.Now;
    regular1.extraString = "extra";

    currentBunch.Add(0, new List<ISomeInterface>());
    currentBunch[0].Add(lite1);

    currentBunch.Add(1, new List<ISomeInterface>());
    currentBunch[1].Add(regular1);

    WriteMe.Write(currentBunch);
}

[DataContract]
public class ISomeInterface
{
    [DataMember]
    public DateTime Date { set; get; }
}