How to Resolve the 'The variable name '@' Has Already Been Declared' SQL Error?

The Variable Name '@' Has Already Been Declared: Resolving Parameter Redundancy

When attempting to execute SQL code, you may encounter the error "The variable name '@LockState' has already been declared. Variable names must be unique within a query batch or stored procedure." This issue arises due to the inclusion of multiple instances of the same parameter in a loop.

To resolve this error, you can either clear the parameters after each loop iteration or add the parameters before the loop.

Clearing Parameters After Each Loop Iteration

for (long counter = from; counter <= to; counter++) {
    // Clear parameters before adding new ones
    rwd.command.Parameters.Clear();

    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;
    rwd.connection.Open();
    rwd.command.ExecuteScalar();
    rwd.connection.Close();
}

Adding Parameters Before Loop and Assigning Values Within 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++) {
    string upd = "update card set LockState=@lockstate, card_descr=@card_descr where [cardNumber] = N'{0}'";
    rwd.command.CommandText = upd;

    // Assign values to parameters within the loop
    rwd.command.Parameters["@LockState"].Value = 1;
    rwd.command.Parameters["@card_descr"].Value = txt_desc2.Text;

    rwd.connection.Open();
    rwd.command.ExecuteScalar();
    rwd.connection.Close();
}

By implementing either of these solutions, you can effectively resolve the parameter redundancy issue and ensure the successful execution of your SQL code.

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