Try using the parameter instead, like this:
using (SqlCommand cmd = new SqlCommand())
{
cmd.CommandText = "Select Distinct Supplier From Material Where Material=@material;";
cmd.Parameters.AddWithValue("@material", comboBoxEdit2.Text);
cmd.Connection = sqlconn;
sqlconn.Open();
using (SqlDataReader reader = cmd.ExecuteReader())
{
while (reader.Read())
{
this.comboBoxEdit3.Properties.Items.Add((string)reader[0]);
}
}
sqlconn.Close();
}