Why does adding a new value to list<> overwrite previous values in the list<>

Question

Following a few tutorials and such I was able to successfully create a collection class which inherits the functionality needed to create a DataTable which can be passed to a Sql Server's stored procedure as a table value parameter. Everything seems to be working well; I can get all of the rows added and it looks beautiful. However, upon closer inspection I notice that when I add a new row, the data for all of the previous rows is overwritten with the value for the new row. So if I have a row with a string value of "foo" and I add a second row with the value "bar", the second row will be inserted (making a DataTable with two rows) but both rows will have the value "bar". Can anyone see why this would be? Here is some of the code, which works but has been a bit simplified (the Tag class has been reduced for ease of explanation).

The following is the Collection class's:

using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;
using System.Web;
using Microsoft.SqlServer.Server;

namespace TagTableBuilder
{
public class TagCollection : List<Tag>, IEnumerable<SqlDataRecord>
{
    IEnumerator<SqlDataRecord> IEnumerable<SqlDataRecord>.GetEnumerator()
    {
        var sdr = new SqlDataRecord(
            new SqlMetaData("Tag", SqlDbType.NVarChar)
            );

        foreach (Tag t in this)
        {
            sdr.SetSqlString(0, t.tagName);

            yield return sdr;
        }
    }
}

public class Tag
{
    public string tagName { get; set; }
}
}

These are called as follows:

//Create instance of collection
TagCollection tags = new TagCollection();

//Create instance of object
Tag _tag = new Tag();

foreach (string t in tagList)
{
    //Add value to class propety 
    _tag.tagName = t;
    //Add class to collection, this is where all previously added rows are overwritten
    tags.Add(_tag);
}

Answer 1

You're using the same instance of the Tag object inside the loop, so each update to the tagName is to the same reference. Move the declaration inside the loop to get a fresh object on each pass of the loop:

foreach (string t in tagList)
{
    Tag _tag = new Tag();

    //Add value to class propety 
    _tag.tagName = t;
    //Add class to collection, this is where all previously added rows are overwritten
    tags.Add(_tag);
}

Also notice that I updated the last line to add _tag instead of mTag as I don't see this defined anywhere.

Answer 2

In the loop where you add the tags to the collection, you're using the same object instance of Tag. Essentially, you're setting a Tag's name to the first value in tagList and adding it to the collection, then you're changing that same Tag's name to the second value in tagList and adding it again to the collection.

Your collection of Tags contains several references to the same Tag object! Instantiate _tag inside the for loop each time before setting the Tag's name and adding it to the collection.