The parameterized query expects the parameter which was not supplied

Avoiding "parameterless" tragedy: handling nulls

When working with parameters, especially when they're tied to user inputs like text boxes, ensure you check for null values. When a null is acceptable, you will need to use DBNull.Value to make your null value palatable to the database:

// If textbox is starving, feed it DBNull
if (string.IsNullOrEmpty(textbox.Text))
{
    command.Parameters.AddWithValue("@ParameterName", DBNull.Value);
}
else
{
    command.Parameters.AddWithValue("@ParameterName", textbox.Text);
}


To match the data type you're assigning, use SqlDbType for a perfect blend:

command.Parameters.Add("@StringParameter", SqlDbType.VarChar).Value = myString;


Doing this means you're raising a ward against SQL injection attacks, keeping your data integrity pure, and paving the way for smoother operation of your database interactions.

Real-time charmer: handling dynamic inputs

Dealing with dynamic user inputs, especially in a TextBox's TextChanged event, requires you to clear preceding results and regenerate based on new input. This calls for a clever way of supplying parameters dynamically:

private void TextBox_TextChanged(object sender, EventArgs e)
{
    // Clear old magic, before casting new one
    myListBox.Clear();

    using (SqlCommand command = new SqlCommand(query, connection))
    {
        // Assign parameter and propagate the magic with new input
        command.Parameters.AddWithValue("@Parameter", TextBox.Text ?? DBNull.Value);
        // Ignite the execution of the updated SQL spell
        // ...
    }
}


Switching off user-added rows in data-bound controls like DataGridView will stave off confusions and potential errors from unsupplied parameters due to incomplete rows.

Become a parameter wizard – tailor your code

CallingConvention – Null delegate

When converting object layers null values to something database-friendly, you should pawn them off as DBNull.Value:

// Shape-shift your Null to DBNull for smoother migration
command.Parameters.AddWithValue("@NullableParameter", myObject.SomeProperty ?? DBNull.Value);


Giving parameters a type – MonoBehaviour

Gift-wrap your data types by using typed parameters like SqlDbType.VarChar and present it to your parameterized query like:

// Package your string properly
command.Parameters.Add("@StringParameter", SqlDbType.VarChar, 50).Value = myStringValue;


Avoid Merlin's error – Check before you leap

Incorporate an if statement to authenticate the validity of the input, a mystical guard stance before the challenge of query hits the database engine:

// To chant the spell or not, that's the question!
if (!string.IsNullOrWhiteSpace(TextBox.Text))
{
    // Execute command and release the magic!
}


Parameter's riddle – The match game

To banish poor, unsuspected mismatches that lead to errors, confirm that the parameters are properly paired with the clauses in SQL CommandText. Review your spellings and employ accurate parameters in your code.