If you are using some legacy SQL in C# you’ll occasionally need to write a WHERE IN.
Yon don’t want to end up writing something like the below, either directly or through some sort of loop.

string cmdText = "SELECT * FROM Members WHERE MemberId IN (100, 200, 300, 400);
SqlCommand cmd = new SqlCommand(cmdText);

Instead you should parameterize the sql. This involves two steps.

1. Add parameters to the sql string.
2. Add the parameters to SqlCommand.

Add a class to your project like SqlWhereInParamBuilder, I chose to use a static method and an extension method, but you can do it in any number of other ways.

using System.Collections.Generic;
using System.Data.SqlClient;
using System.Linq;

namespace SqlWhereInParameters
{
    public static class SqlWhereInParamBuilder
    {
        public static string BuildWhereInClause<T>(string partialClause, string paramPrefix, IEnumerable<T> parameters)
        {
            string[] parameterNames = parameters.Select(
                (paramText, paramNumber) => "@" + paramPrefix + paramNumber.ToString())
                .ToArray();

            string inClause = string.Join(",", parameterNames);
            string whereInClause = string.Format(partialClause.Trim(), inClause);
            
            return whereInClause;
        }

        public static void AddParamsToCommand<T>(this SqlCommand cmd, string paramPrefix, IEnumerable<T> parameters)
        {
            string[] parameterValues = parameters.Select((paramText) => paramText.ToString()).ToArray();

            string[] parameterNames = parameterValues.Select(
                (paramText, paramNumber) => "@" + paramPrefix + paramNumber.ToString()
                ).ToArray();

            for (int i = 0; i < parameterNames.Length; i++)
            {
                cmd.Parameters.AddWithValue(parameterNames[i], parameterValues[i]);
            }
        }
    }
}

This is the usage

using System.Collections.Generic;
using System.Data.SqlClient;

namespace SqlWhereInParameters
{
    class Program
    {
        static void Main(string[] args)
        {
            Program p = new Program();
            p.RunCommandBulderForFullSelectExample();
        }

        private void RunCommandBulderForFullSelectExample()
        {
            List<long> memberIds = new List<long> { 100, 200, 300, 400 };
            string parameterPrefix = "MemberId";
            string cmdText = "SELECT * FROM Member WHERE Name IN ({0})";

            cmdText = SqlWhereInParamBuilder.BuildWhereInClause(cmdText, parameterPrefix, memberIds);

            SqlCommand cmd = new SqlCommand(cmdText);
            cmd.AddParamsToCommand(parameterPrefix, memberIds);
        }
    }
}