Unique constraint being violated on the wrong column when using SqlBulkCopy

Question

I'm trying to use SqlBulkCopy as a way of doing multiple INSERTs at once but for some reason, I'm getting a unique constraint violation when running WriteToServer(DataTable). The odd thing about this SqlException is it's saying that .

My table schema:

CREATE TABLE Product (
  ID   INT IDENTITY (1, 1) PRIMARY KEY,
  Name NVARCHAR(450) UNIQUE NOT NULL, -- Unique constraint being called
  BulkInsertID NCHAR(6) -- Column the constraint is being called on
);

The only reason I can think of as to why this is happening is because I mixed up the column names when assigning them inside the DataColumns but I checked them multiple times and I cannot find any issues with them.

Minimal, Complete and Verifiable Example:

class Program
{
    private static SqlConnection connection;
    private static string connectionURL = "Server=ASUS-X750JA\\DIRECTORY;Database=directory;Integrated Security=True;";
    private static Random _random = new Random();

    public static SqlConnection openConnection()
    {
        connection = new SqlConnection(connectionURL);
        connection.Open();
        Console.WriteLine("Opened connection to DB");
        return connection;
    }

    public static void closeConnection()
    {
        connection.Close();
        Console.WriteLine("Closed connection to DB");
    }

    static void Main(string[] args)
    {
        List<string> productNames = new List<string>();
        productNames.Add("Diamond");
        productNames.Add("Gold");
        productNames.Add("Silver");
        productNames.Add("Platinum");
        productNames.Add("Pearl");
        addProducts(productNames);
    }

    private static void addProducts(List<string> productNames)
    {
        const string tableName = "Product";
        DataTable table = new DataTable(tableName);

        string bulkInsertID;
        do
        {
            bulkInsertID = generateID();
        } while (isDuplicateBulkInsertID(tableName, bulkInsertID));

        DataColumn nameColumn = new DataColumn("Name");
        nameColumn.Unique = true;
        nameColumn.AllowDBNull = false;

        DataColumn bulkInsertIDColumn = new DataColumn("BulkInsertID");
        bulkInsertIDColumn.Unique = false;
        bulkInsertIDColumn.AllowDBNull = true;

        table.Columns.Add(nameColumn);
        table.Columns.Add(bulkInsertIDColumn);

        foreach (string productName in productNames)
        {
            DataRow row = table.NewRow();
            row[nameColumn] = productName;
            row[bulkInsertIDColumn] = bulkInsertID;
            table.Rows.Add(row);
        }

        using (SqlConnection connection = openConnection())
        {
            using (SqlBulkCopy bulkCopy = new SqlBulkCopy(connection))
            {
                bulkCopy.DestinationTableName = table.TableName;
                bulkCopy.WriteToServer(table);
            }
        }
    }

    /// <summary>
    /// Generates random 6-character string but it's not like GUID so may need to check for duplicates
    /// </summary>
    /// <returns></returns>
    public static string generateID()
    {
        char[] _base62chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz".ToCharArray();
        int length = 6;

        var sb = new StringBuilder(length);

        for (int i = 0; i < length; i++)
            sb.Append(_base62chars[_random.Next(62)]);

        return sb.ToString();
    }

    public static bool isDuplicateBulkInsertID(string tableName, string bulkInsertID)
    {
        string query = string.Format("SELECT BulkInsertID FROM {0} WHERE BulkInsertID = @bulkinsertid", tableName);
        SqlCommand command = new SqlCommand(query, openConnection());
        SqlParameter bulkInsertIDParam = new SqlParameter("@bulkinsertid", SqlDbType.NChar, bulkInsertID.Length);
        bulkInsertIDParam.Value = bulkInsertID;

        command.Parameters.Add(bulkInsertIDParam);
        command.Prepare();
        Task<SqlDataReader> asyncTask = command.ExecuteReaderAsync();
        SqlDataReader reader = asyncTask.Result;
        bool isDuplicate = reader.HasRows;

        closeConnection();
        return isDuplicate;
    }
}

The unique constraint shown in the screenshot belongs to the Name column but the duplicate key value is being sent to the BulkInsertID column and I don't know why the error is being thrown.

EDIT: I just changed my schema to use uniqueidentifier as the bulkInsertID column and changed row[bulkInsertIDColumn] = bulkInsertID to row[bulkInsertIDColumn] = Guid.NewGuid().ToString(). When I reran my code, I found that the generated GUID ran but when I looked at the table, the GUID was in the name column. So I can conclude it's not a server issue but a problem in the program.

Answer 1

Because you have a identity column bulk insert is trying to insert nameColumn in to ID (and ignoring it because the column is a identity column) and bulkInsertIDColumn in to Name. Just add the following to your insert to tell it to go to the correct columns.

using (SqlBulkCopy bulkCopy = new SqlBulkCopy(connection))
{
    bulkCopy.ColumnMappings.Add("Name", "Name"); //NEW
    bulkCopy.ColumnMappings.Add("BulkInsertID", "BulkInsertID"); //NEW
    bulkCopy.DestinationTableName = table.TableName;
    bulkCopy.WriteToServer(table);
}

The other option is add a ID column to table and just don't put any values in it.

DataColumn idColumn = new DataColumn("ID");

DataColumn nameColumn = new DataColumn("Name");
//nameColumn.Unique = true; //SqlBulkCopy does not care about these settings.
//nameColumn.AllowDBNull = false;

DataColumn bulkInsertIDColumn = new DataColumn("BulkInsertID");
//bulkInsertIDColumn.Unique = false;
//bulkInsertIDColumn.AllowDBNull = true;

table.Columns.Add(ID);
table.Columns.Add(nameColumn);
table.Columns.Add(bulkInsertIDColumn);

foreach (string productName in productNames)
{
    DataRow row = table.NewRow();
    //We don't do anything with row[idColumn]
    row[nameColumn] = productName;
    row[bulkInsertIDColumn] = bulkInsertID;
    table.Rows.Add(row);
}

Answer 2

Looks like it's throwing UNIQUE constraint violation for column BulkInsertID though from posted table schema don't see it's marked with that constraint and in your code I see you have bulkInsertIDColumn.Unique = false;. Are you sure you are not setting it to true anywhere else.

BTW, to me looks it's throwing that exception cause you are trying to create a new instance of Random() in loop as seen below pointed code block

    do
    {
        bulkInsertID = generateID(); //calling method generateID
    } while (isDuplicateBulkInsertID(tableName, bulkInsertID));

Where as in generateID() you are creating new instance of Random class

public static string generateID()
{
   ........
    Random _random = new Random(); // creating new instance every time