Why Does My SQL Query Throw a 'Variable Name Already Declared' Error?

Variable Name Conflict in SQL Queries

Your error message, "The variable name '@' has already been declared. Variable names must be unique within a query batch or stored procedure," indicates a conflict in variable naming within your SQL code.

In the provided code snippet, the issue arises when trying to use the same variable name, "@LockState," multiple times in a loop within the "btn_lock2_Click" event handler. SQL requires unique variable names within a query or stored procedure to prevent confusion and ambiguity.

Solution 1: Clear Parameters after Each Loop Iteration

One way to resolve the conflict is to clear the parameters collection before each loop iteration. This ensures that the same parameter name is not added multiple times to the collection.

for (long counter = from; counter <= to; counter )
{

rwd.command.Parameters.Clear(); // Clear the parameters collection before adding new parameters

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;
...

}

Solution 2: Define Parameters Before Loop

Alternatively, you can define the parameters before the loop and then update their values within each iteration. This approach ensures that the parameters exist within the collection before the loop starts, eliminating the need for clearing.

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;

rwd.command.Parameters["@LockState"].Value = 1;
rwd.command.Parameters["@card_descr"].Value = txt_desc2.Text;

...

}

By using either of these solutions, you can ensure that your SQL queries have unique variable names, preventing the "already declared" error.

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