尝试在 C# Winform 中查询 PostgreSQL 数据库时，ExecuteReader 出现 Npgsql.PostgresException 错误

Question

I'm trying to get a list of strings from a database query and put those in a ComboBox.

The code that I'm attempting to do that with starts with the following on my from in a DataGridVew Cell Click event:

// Bind combobox to dgv and than add list of New Values to combobox
DataGridViewComboBoxCell cboNewValueList = new DataGridViewComboBoxCell();

// Get fields to build New Value query
List<string> lsNewValuesResult = new List<string>();
string strCategory = dtCategories.Rows[e.RowIndex][1].ToString();
string strCompanyName = cboSelectCompany.Text;
string strQueryGetNewValuesValidationInfo = "SELECT validationdb, validationtable, validationfield, validationfield2, validationvalue2" +
                                        " FROM masterfiles.categories" +
                                        " WHERE category = @category";

// Pass validation info query to db class and return list of New Values
db getListOfNewValues = new db();
lsNewValuesResult = getListOfNewValues.GetNewValuesList(strQueryGetNewValuesValidationInfo, strCategory, strCompanyName);

//Populate the combobox with the list of New Values
foreach (string strListItem in lsNewValuesResult)
{
    cboNewValueList.Items.Add(strListItem);
}

I need to use the above query to find out what table and fields I need to for the query to populate lsNewValuesResult .

In GetNewValuesList I have this code:

public List<string> GetNewValuesList(string strValidationInfoQuery, string strCategory, string strCompanyName)
{
    List<string> lsValidationInfo = new List<string>();
    List<string> lsNewValuesList = new List<string>();

    using (NpgsqlConnection conn = new NpgsqlConnection(connString))
    using (NpgsqlCommand cmd = new NpgsqlCommand(strValidationInfoQuery, conn))
    {
        cmd.Parameters.Add(new NpgsqlParameter("@cateogry", strCategory));
        conn.Open();

        using (NpgsqlDataReader reader = cmd.ExecuteReader())
        {
            while (reader.Read())
            {
                int intReaderIndex;
                for (intReaderIndex = 0; intReaderIndex <= reader.FieldCount - 1; intReaderIndex++)
                {
                    // reader indexes 3 & 4 correspond to categories.validationfield2 and validationvalue2, which can be null
                    if (string.IsNullOrEmpty(reader[intReaderIndex].ToString()))
                    {
                        lsValidationInfo.Add("");
                    }
                    else
                    {
                        lsValidationInfo.Add(reader.GetString(intReaderIndex));
                    }
                    Console.WriteLine("reader index " + intReaderIndex + ": " + reader.GetString(intReaderIndex));
                }
            }
        }
    }

    // Use the items in lsValidationInfo to build the query to get the New Values
}

On the ExecuteReader() line, though I'm getting an Npgsql.PostgresException, "operator does not exist: @ character varying'".

What's puzzling me is that I modeled this after another block of code in the same project that does the same thing successfully. It was pretty much a copy/paste and then change the parameter and object names for the new function.

Answer 1

So i think problem is with following line:

cmd.Parameters.Add(new NpgsqlParameter("@cateogry", strCategory));

Try replacing with following:

cmd.Parameters.AddWithValue("category", strCategory);

Answer 2

Just another way to write it, with this you can also provide other attributes like Parameter direction like OUT and INOUT as well-

cmd.Parameters.Add(new NpgsqlParameter("@cateogry", NpgsqlTypes.NpgsqlDbType.Varchar){ Value = strCategory,Direction= ParameterDirection.Input });

Thanks!