c# Error converting data type varchar to numeric.

Question

my code

 

projeservices = new HProje();

projeservices.insert("INSERT INTO ProjeEkle(IsinAdi,Yuklenici,IhaleBedeli,IhaleTarihi,SozlesmeTarihi,IsinSuresi,TeslimTarihi,BaslamaTarihi,BitisTarihi)values('" + txtisinadi.Text + "','" + txtyuklenici.Text + "','" +ntxtihalebedeli.Text + "','" +dtpihaletarihi.Text + "','" + dtpsozlesmetarihi.Text + "','" +ntxtisinsuresi.Text + "','" + dtptxtteslim.Text + "','" + dtptxtbaslama.Text + "','" + dtptxtbitis.Text + "')");

 

 

my ProjeEkle table

 

Id int Unchecked

IsinAdi nvarchar(200) Unchecked

Yuklenici nvarchar(200) Unchecked

IhaleBedeli decimal(18, 3) Unchecked

IhaleTarihi datetime Unchecked

SozlesmeTarihi datetime Unchecked

IsinSuresi int Unchecked

TeslimTarihi datetime Unchecked

BaslamaTarihi datetime Unchecked

BitisTarihi datetime Unchecked

Unchecked

 

Can you help title also is the reason I get the error