Why Am I Getting the 'The Variable Name '@' Has Already Been Declared' Error in My SQL Queries?

"The Variable Name '@' Has Already Been Declared" Error in SQL Queries

When executing SQL queries, it's crucial to ensure that variable names declared within the query batch or stored procedure are unique. This error often occurs when using variable substitution with the '@' prefix.

Cause of the Error

In the provided code, the second instance of '@LockState' in the parameter collection triggers the error. This is because each parameter must have a unique name within the same query.

Resolution

To resolve this error, implement one of the following solutions:

• Clear the Parameters Collection after Each Iteration:

Use rwd.command.Parameters.Clear() after each loop iteration to remove the previously added parameters, ensuring that the next iteration starts with a new set.

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

• Declare Parameters Outside the Loop:

Declare the parameters outside the loop and assign values within the loop. This ensures the parameters are defined only once.

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++)
{
    // ... (remaining code)
    rwd.command.Parameters["@LockState"].Value = 1;
    rwd.command.Parameters["@card_descr"].Value = txt_desc2.Text;
}

By following these steps, you can ensure unique variable names in your queries, preventing the "The variable name '@' has already been declared" error and ensuring seamless execution.

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