How to Fix 'The variable name '@' has already been declared' Error in SQL Query Batch?

Variable Name Conflict in SQL Query Batch

When executing SQL queries, it's crucial to ensure uniqueness in variable names within a batch or stored procedure. The error "The variable name '@' has already been declared" indicates that multiple variables with the same name were defined.

To resolve this, focus on the code block provided:

private void btn_lock2_Click(object sender, EventArgs e)
{
    // ...
    for (long counter = from; counter <= to; counter++)
    {
        string upd = "update card set LockState=@lockstate, card_descr=@card_descr where [cardNumber] = N'{0}'";
        rwd.command.CommandText = upd;
        rwd.command.Parameters.Add(new SqlParameter("@LockState", SqlDbType.NVarChar)).Value = 1;
        rwd.command.Parameters.Add(new SqlParameter("@card_descr", SqlDbType.NVarChar)).Value = txt_desc2.Text;
        // ...
    }
}

The mistake lies in repeatedly adding parameters within the loop. Instead, move the parameter creation outside the loop:

// Moved outside the loop
rwd.command.Parameters.Add(new SqlParameter("@LockState", SqlDbType.NVarChar));
rwd.command.Parameters.Add(new SqlParameter("@card_descr", SqlDbType.NVarChar));

for (long counter = from; counter <= to; counter++)
{
    // ...
    rwd.command.Parameters["@LockState"].Value = 1;
    rwd.command.Parameters["@card_descr"].Value = txt_desc2.Text;
    // ...
}

Alternatively, consider clearing the parameters within the loop:

for (long counter = from; counter <= to; counter++)
{
    rwd.command.Parameters.Clear();
    // ...
}

By following these recommendations, you can eliminate the error and ensure the uniqueness of variable names in your SQL queries.

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