Best way to remove the last character from a string built with stringbuilder

The simplest and most efficient way is to perform this command:

data.Length--;

by doing this you move the pointer (i.e. last index) back one character but you don't change the mutability of the object. In fact, clearing a StringBuilder is best done with Length as well (but do actually use the Clear() method for clarity instead because that's what its implementation looks like):

data.Length = 0;

again, because it doesn't change the allocation table. Think of it like saying, I don't want to recognize these bytes anymore. Now, even when calling ToString(), it won't recognize anything past its Length, well, it can't. It's a mutable object that allocates more space than what you provide it, it's simply built this way.

Use the following after the loop.

.TrimEnd(',')

or simply change to

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

Just use

string.Join(",", yourCollection)

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

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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();
        }
    }
}

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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.