Just use

string.Join(",", yourCollection)

This way you don't need the StringBuilder and the loop.

Long addition about async case. As of 2019, it's not a rare setup when the data are coming asynchronously.

In case your data are in async collection, there is no string.Join overload taking IAsyncEnumerable<T>. But it's easy to create one manually, hacking the code from string.Join:

public static class StringEx
{
    public static async Task<string> JoinAsync<T>(string separator, IAsyncEnumerable<T> seq)
    {
        if (seq == null)
            throw new ArgumentNullException(nameof(seq));

        await using (var en = seq.GetAsyncEnumerator())
        {
            if (!await en.MoveNextAsync())
                return string.Empty;

            string firstString = en.Current?.ToString();

            if (!await en.MoveNextAsync())
                return firstString ?? string.Empty;

            // Null separator and values are handled by the StringBuilder
            var sb = new StringBuilder(256);
            sb.Append(firstString);

            do
            {
                var currentValue = en.Current;
                sb.Append(separator);
                if (currentValue != null)
                    sb.Append(currentValue);
            }
            while (await en.MoveNextAsync());
            return sb.ToString();
        }
    }
}

If the data are coming asynchronously but the interface IAsyncEnumerable<T> is not supported (like the mentioned in comments SqlDataReader), it's relatively easy to wrap the data into an IAsyncEnumerable<T>:

async IAsyncEnumerable<(object first, object second, object product)> ExtractData(
        SqlDataReader reader)
{
    while (await reader.ReadAsync())
        yield return (reader[0], reader[1], reader[2]);
}

and use it:

Task<string> Stringify(SqlDataReader reader) =>
    StringEx.JoinAsync(
        ", ",
        ExtractData(reader).Select(x => $"{x.first} * {x.second} = {x.product}"));

In order to use Select, you'll need to use nuget package System.Interactive.Async. Here you can find a compilable example.

How about this..

string str = "The quick brown fox jumps over the lazy dog,";
StringBuilder sb = new StringBuilder(str);
sb.Remove(str.Length - 1, 1);

Use the following after the loop.

.TrimEnd(',')

or simply change to

string commaSeparatedList = input.Aggregate((a, x) => a + ", " + x)

I prefer manipulating the length of the stringbuilder:

data.Length = data.Length - 1;

Gotcha!!

Most of the answers on this question won't work if you use AppendLine like below:

var builder = new StringBuilder();
builder.AppendLine("One,");
builder.Length--; // Won't work
Console.Write(builder.ToString());

builder = new StringBuilder();
builder.AppendLine("One,");
builder.Length += -1; // Won't work
Console.Write(builder.ToString());

builder = new StringBuilder();
builder.AppendLine("One,");
Console.Write(builder.TrimEnd(',')); // Won't work

Fiddle Me

WHY??? @(&**(&@!!

The issue is simple but took me a while to figure it out: Because there are 2 more invisible characters at the end CR and LF (Carriage Return and Line Feed). Therefore, you need to take away 3 last characters:

var builder = new StringBuilder();
builder.AppendLine("One,");
builder.Length -= 3; // This will work
Console.WriteLine(builder.ToString());

In Conclusion

Use Length-- or Length -= 1 if the last method you called was Append. Use Length -= 3 if you the last method you called AppendLine.

I recommend, you change your loop algorithm:

• Add the comma not AFTER the item, but BEFORE
• Use a boolean variable, that starts with false, do suppress the first comma
• Set this boolean variable to true after testing it

You should use the string.Join method to turn a collection of items into a comma delimited string. It will ensure that there is no leading or trailing comma, as well as ensure the string is constructed efficiently (without unnecessary intermediate strings).

The most simple way would be to use the Join() method:

public static void Trail()
{
    var list = new List<string> { "lala", "lulu", "lele" };
    var data = string.Join(",", list);
}

If you really need the StringBuilder, trim the end comma after the loop:

data.ToString().TrimEnd(',');

Yes, convert it to a string once the loop is done:

String str = data.ToString().TrimEnd(',');

You have two options. First one is very easy use Remove method it is quite effective. Second way is to use ToString with start index and end index (MSDN documentation)

Similar SO question here.

I liked the using a StringBuilder extension method.

RemoveLast Method

Simply shortens the stringbuilder length by 1;

 StringBuilder sb = new StringBuilder();
 sb.Length--;

i know this is not the effective way as it translates to sb = sb-1;

Alternative Effective solution

sb.Remove(starting_index, how_many_character_to_delete);

for our case it would be

sb.Remove(sb.length-1,1)