SQL INSERT failed, I can't find the issue

Question

I'm trying to insert something into a SQL server (using c#) and when I try to it says it requires the (in the query) given parameter and that it can't find it while it is declared.

"Additional information: The parameterized query '(@art varchar(8000),@oms varchar(8000),@rem varchar(8000),@artdk' expects the parameter '@art', which was not supplied."

I checked parameter amount, the database connection and I tried to use another insert command I have as reference but I couldn't get it to work. and I couldn't find the same sort of problem on the internet yet. if someone could help me, it would be much appreciated.

UPDATE: I changed the artnr.selectedvalue to artnr.text and that sort of got me some progress but now I get this error

UPDATE2: seems that if I fill every box it tries to insert it works.. yet it does allow null in all database cell's so the issue has been found. just got to work out which one('s) cant have empty.

An unhandled exception of type 'System.Data.SqlClient.SqlException' occurred in System.Data.dll

Additional information: Error converting data type varchar to numeric.

it gives this error when I put in all numeric or all text

SqlCommand slinkoopadd = new SqlCommand(@"insert into ART (ART ,OMS ,REM ,ARTDK ,TYPE ,MAG ,PROGRAM ,EH1 ,LEV ,LTD ,INK ,KOR ,SGR ,EH2 ,EF ,VALUTA ,CRNI )
                                                   values (@art,@oms,@rem,@artdk,@type,@mag,@program,@eh1,@lev,@ltd,@ink,@kor,@sgr,@eh2,@ef,@valuta,@crni);", Connectie.connMEVO_ART);
        #region parameters
        slinkoopadd.Parameters.Add("@art", SqlDbType.VarChar).Value = this.artnr.SelectedValue;
        slinkoopadd.Parameters.Add("@oms", SqlDbType.VarChar).Value = this.tekstinkoopoms.Text;
        slinkoopadd.Parameters.Add("@rem", SqlDbType.VarChar).Value = this.tekstinkoopopmerk.Text;
        slinkoopadd.Parameters.Add("@artdk", SqlDbType.VarChar).Value = this.tekstinkoopnummerlev.Text;
        slinkoopadd.Parameters.Add("@type", SqlDbType.VarChar).Value = this.tekstinkooparttype.Text;
        slinkoopadd.Parameters.Add("@mag", SqlDbType.VarChar).Value = this.tekstinkoopmagazijnloc.Text;
        slinkoopadd.Parameters.Add("@program", SqlDbType.VarChar).Value = this.tekstinkoopinternopmerk.Text;
        slinkoopadd.Parameters.Add("@eh1", SqlDbType.VarChar).Value = this.tekstinkoopeenheid.Text;
        slinkoopadd.Parameters.Add("@lev", SqlDbType.VarChar).Value = this.tekstinkoopstandleveran.Text;
        slinkoopadd.Parameters.Add("@ltd", SqlDbType.VarChar).Value = this.tekstinkooplevertijd.Text;
        slinkoopadd.Parameters.Add("@ink", SqlDbType.VarChar).Value = this.tekstinkoopbrutoprijs.Text;
        slinkoopadd.Parameters.Add("@kor", SqlDbType.VarChar).Value = this.tekstinkoopkorting.Text;
        slinkoopadd.Parameters.Add("@sgr", SqlDbType.VarChar).Value = this.tekstinkoopserievoorraad.Text;
        slinkoopadd.Parameters.Add("@eh2", SqlDbType.VarChar).Value = this.tekstinkoopgebruikeh.Text;
        slinkoopadd.Parameters.Add("@ef", SqlDbType.VarChar).Value = this.textinkoopehfactor.Text;
        //slinkoopadd.Parameters.Add("@", SqlDbType.VarChar).Value = this.artnr.Text;//perc. voor vracht
        slinkoopadd.Parameters.Add("@valuta", SqlDbType.VarChar).Value = this.tekstinkoopvaluta.Text;
        slinkoopadd.Parameters.Add("@crni", SqlDbType.VarChar).Value = this.tekstinkoopcrni.Text;
        //slinkoopadd.Parameters.Add("@", SqlDbType.VarChar).Value = this.artnr.Text;//extra kosten
        //slinkoopadd.Parameters.Add("@", SqlDbType.VarChar).Value = this.artnr.Text;//bestelgrootte afroep
        //slinkoopadd.Parameters.Add("@", SqlDbType.VarChar).Value = this.artnr.Text;//prognose jaarverbruik
        //slinkoopadd.Parameters.Add("@", SqlDbType.VarChar).Value = this.artnr.Text;//levertijd nieuwe afr
        #endregion
        drART = slinkoopadd.ExecuteReader();
        MessageBox.Show("Opgeslagen!");
        fillbox();
        while (drART.Read())
        { }
        slinkoopadd.Dispose();

Answer 1

i changed some small things in the query and i made sure some of the textboxes get a default value if it doesnt have anything in it so now it works. this is the current query that works.

if (tekstinkoopcrni.Text == "1") { } else if (tekstinkoopcrni.Text == "0") { } 
else { tekstinkoopcrni.Text = "0"; }
if (tekstinkoopkorting.Text == "") { tekstinkoopkorting.Text = "0"; }
if (tekstinkoopserievoorraad.Text == "") { tekstinkoopserievoorraad.Text = "0"; }
if (tekstinkoopstandleveran.Text == "") { tekstinkoopstandleveran.Text = "9999"; }
if (tekstinkooplevertijd.Text == "") { tekstinkooplevertijd.Text = "0"; }

SqlCommand slinkoopadd = new SqlCommand(@"insert into ART (ART ,OMS ,REM ,ARTDK ,TYPE ,
MAG ,PROGRAM ,EH1 ,INK ,KOR ,SGR ,EH2 ,EF ,VALUTA ,CRNI  ,LEV ,LTD )
values (@art,@oms,@rem,@artdk,@type,@mag,@program,@eh1,@ink,@kor,@sgr,@eh2,@ef,
@valuta,@crni, @lev,@ltd);", Connectie.connMEVO_ART);                                   

#region parameters
slinkoopadd.Parameters.Add("@art", SqlDbType.VarChar).Value = this.artnr.Text;
slinkoopadd.Parameters.Add("@oms", SqlDbType.VarChar).Value = this.tekstinkoopoms.Text;
slinkoopadd.Parameters.Add("@rem", SqlDbType.VarChar).Value = this.tekstinkoopopmerk.Text;
slinkoopadd.Parameters.Add("@artdk", SqlDbType.VarChar).Value = this.tekstinkoopnummerlev.Text;
slinkoopadd.Parameters.Add("@type", SqlDbType.VarChar).Value = this.tekstinkooparttype.Text;
slinkoopadd.Parameters.Add("@mag", SqlDbType.VarChar).Value = this.tekstinkoopmagazijnloc.Text;
slinkoopadd.Parameters.Add("@program", SqlDbType.VarChar).Value = this.tekstinkoopinternopmerk.Text;
slinkoopadd.Parameters.Add("@eh1", SqlDbType.VarChar).Value = this.tekstinkoopeenheid.Text;
slinkoopadd.Parameters.Add("@lev", SqlDbType.VarChar).Value = this.tekstinkoopstandleveran.Text;
slinkoopadd.Parameters.Add("@ltd", SqlDbType.VarChar).Value = this.tekstinkooplevertijd.Text;
slinkoopadd.Parameters.Add("@ink", SqlDbType.VarChar).Value = this.tekstinkoopbrutoprijs.Text;
slinkoopadd.Parameters.Add("@kor", SqlDbType.VarChar).Value = this.tekstinkoopkorting.Text;
slinkoopadd.Parameters.Add("@sgr", SqlDbType.VarChar).Value = this.tekstinkoopserievoorraad.Text;
slinkoopadd.Parameters.Add("@eh2", SqlDbType.VarChar).Value = this.tekstinkoopgebruikeh.Text;
slinkoopadd.Parameters.Add("@ef", SqlDbType.VarChar).Value = this.textinkoopehfactor.Text;
slinkoopadd.Parameters.Add("@valuta", SqlDbType.VarChar).Value = this.tekstinkoopvaluta.Text;
slinkoopadd.Parameters.Add("@crni", SqlDbType.VarChar).Value = this.tekstinkoopcrni.Text;
#endregion
drART = slinkoopadd.ExecuteReader();
MessageBox.Show("Artikel opgeslagen!");
fillbox();
while (drART.Read())
{ }
slinkoopadd.Dispose(); 

Answer 2

drART = slinkoopadd.ExecuteReader();

You don't expect to read something. You want to insert, that is not a query:

var result = slinkoopadd.ExecuteNonQuery();

Answer 3

Try it like this?

slinkoopadd.Parameters.Add("@art", SqlDbType.VarChar);
slinkoopadd.Parameters["@art"].Value = this.artnr.SelectedValue;

I'm not sure you can pipe the call like that.