Question

There is an exception message when I run the website

” Unable to cast object of type ‘System.DBNull’ to type
‘System.String’.”

emereging from the ‘value’ part of 'return (string)c.Parameters["@vat"].Value;‘ in the below code. I checked the stored procedure to see if it executes the query, it does and its output is a string as well. Can Anyone help..??

namespace CamOnlineAccess
{
  public class Utilities
  {
    public SqlConnection GetConnection()
    {
        SqlConnection c = new SqlConnection();
        c.ConnectionString = ConfigurationManager.ConnectionStrings["CamOnlineConnectionString"].ConnectionString;
        return c;
    }

    public SqlCommand GetCommandSP(string SPName)
    {
        SqlCommand c = new SqlCommand();
        c.Connection = GetConnection();
        c.CommandType = CommandType.StoredProcedure;
        c.CommandText = SPName;
        c.CommandTimeout = 150;

        return c;
    }
  }
}

public string ViewUserVat(string code)
{
    CamOnlineAccess.Utilities u = new CamOnlineAccess.Utilities();
    SqlCommand c = u.GetCommandSP("dbo.ViewUserVat");

    c.Parameters.AddRange(new System.Data.SqlClient.SqlParameter[] {
        new System.Data.SqlClient.SqlParameter("@code", System.Data.SqlDbType.VarChar,50),
            new System.Data.SqlClient.SqlParameter("@vat",SqlDbType.VarChar, 50, System.Data.ParameterDirection.Output, false, ((byte)(0)), ((byte)(0)), "", System.Data.DataRowVersion.Current, null)});
    c.Parameters["@code"].Value = code;
    c.Connection.Open();
    c.ExecuteScalar();//because we have output parameters
    c.Connection.Close();

    return (string)c.Parameters["@vat"].Value;
 }

STRORED PROCEDURE

 ALTER PROCEDURE [dbo].[ViewUserVat] 
-- Add the parameters for the stored procedure here
@code varchar,
@vat varchar(50) output

    AS
SELECT top 1 @vat=vattable from dbo.portfolio 
where owner=@code

Answer 1

You return directly the parameter value assuming that your stored procedure will always find a record for the parameter @code and the column vattable is never null.

This is not the case as you can see, and you get a DBNull in return.
You should change your code to

 return (c.Parameters["@vat"].Value == DBNull.Value ? 
              null : 
              c.Parameters["@vat"].Value.ToString());

Of course, assuming a null is an allowed/expected return value for the rest of your code.
Otherwise, you can replace the null with another acceptable value (eg string.Empty)