C# Sql Command

What does the SqlCommand look like? The one that is causing the problem that is. You do not show that so we cannot be specific about the solution but there is probably a problem there. Since the error says Text there is probably a (syntax) problem with the way you are creating the text for the command.

Usually data operations are in their own class e.g.

A container

public class Person  
{  
    public long Id { get; set; }  
    public string FirstName { get; set; }  
    public string LastName { get; set; }  
    public long AccountNumber { get; set; }  
}  

Operation class

public class DataOperations  
{  
     
	public static Person GetPerson(int identifier)  
	{  
		Person person = new Person();  
  
		var selectStatement = "SELECT FirstName, LastName, AccountNumber FROM People WHERE Id = @Id";  
		using var cn = new SqlConnection() { ConnectionString = "TODO" };  
		var cmd = new SqlCommand() {Connection = cn, CommandText = selectStatement};  
  
		cmd.Parameters.Add("@Id", SqlDbType.Int).Value = identifier;  
  
		cn.Open();  
		  
		var reader = cmd.ExecuteReader();  
		  
		if (reader.HasRows)  
		{  
			reader.Read();  
			person.Id = identifier;  
			person.FirstName = reader.GetString(0);  
			person.LastName = reader.GetString(1);  
			person.AccountNumber = reader.GetInt64(2);  
		}  
  
		return person;  
  
	}  
}  

Then calling here you can assign property values to controls .Text property for instance.

var person = DataProviderOperations.GetPerson(1);  
var accountAsString = person.AccountNumber.ToString();