using System;

using System.Linq;

using System.Collections.Generic;

​

public enum TestEnum

{

    Value0 = 0,

    Value1,

    Value2,

}

​

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());

    }

}