using System;

using System.Linq;

using System.Collections.Generic;

​

public enum TestEnum

{

    Value0 = 0,

    Value1,

    Value2,

}

​

public struct NotAnEnum: IConvertible

{

        #region IConvertible Implementation

        TypeCode IConvertible.GetTypeCode() { return TypeCode.Object; }

        bool IConvertible.ToBoolean(IFormatProvider provider) { throw new NotImplementedException(); }

        char IConvertible.ToChar(IFormatProvider provider) { throw new NotImplementedException(); }

        sbyte IConvertible.ToSByte(IFormatProvider provider) { throw new NotImplementedException(); }

        byte IConvertible.ToByte(IFormatProvider provider) { throw new NotImplementedException(); }

        short IConvertible.ToInt16(IFormatProvider provider) { throw new NotImplementedException(); }

        ushort IConvertible.ToUInt16(IFormatProvider provider) { throw new NotImplementedException(); }

        int IConvertible.ToInt32(IFormatProvider provider) { throw new NotImplementedException(); }

        uint IConvertible.ToUInt32(IFormatProvider provider) { throw new NotImplementedException(); }

        long IConvertible.ToInt64(IFormatProvider provider) { throw new NotImplementedException(); }

        ulong IConvertible.ToUInt64(IFormatProvider provider) { throw new NotImplementedException(); }

        float IConvertible.ToSingle(IFormatProvider provider) { throw new NotImplementedException(); }

        double IConvertible.ToDouble(IFormatProvider provider) { throw new NotImplementedException(); }

        decimal IConvertible.ToDecimal(IFormatProvider provider) { throw new NotImplementedException(); }

        DateTime IConvertible.ToDateTime(IFormatProvider provider) { throw new NotImplementedException(); }

        string IConvertible.ToString(IFormatProvider provider) { throw new NotImplementedException(); }

        object IConvertible.ToType(Type conversionType, IFormatProvider provider) { throw new NotImplementedException(); }

        #endregion 

}

​

public static class EnumUtils

{

    // Available in C# 7.3

    //public static string ToSqlNullableString<T>(this List<T> list)

    //  where T : Enum

    //{

    //  return string.Join(separator: ",", values: list.Select(_e => Convert.ToInt32(_e)));

    //}

​

    public static string ToSqlNullableString<T>(this List<T> list) where T : struct, IConvertible

    {

        if (!typeof(T).IsEnum)

            throw new ArgumentException("T must be an enumerated type");

        return string.Join(separator: ",", values: list.Select(_e => Convert.ToInt32(_e)));

    }

}

​

public class Program

{

    public static void Main()

    {

        var enumList = new List<TestEnum>

        {

            TestEnum.Value0, TestEnum.Value0,

            TestEnum.Value1, TestEnum.Value1,

            TestEnum.Value2, TestEnum.Value2,

        };

​

        //Console.WriteLine(enumList.ToSqlNullableString());

        Console.WriteLine(enumList.ToSqlNullableString());

        var notAnEnumList = new List<NotAnEnum>

        {

            new NotAnEnum(),

            new NotAnEnum(),

            new NotAnEnum(),

        };

        Console.WriteLine(notAnEnumList.ToSqlNullableString());

    }

}