Incorrect syntax near ','. data insert problems in number field.

In Your Second Case when You leave your Textboxfield (SrNo empty)
then it passes "" to the databse which is incompatible with your Decimal field
sr.No(Decimal(18,0)
So need to check before passing it to the database like below

Decimal Value1;
 if(TextBox1.text=="")
  {
   Value1 = 0.0;
   }
  else
   {
   Value1 = Convert.ToDecimal(TextBox1.text);
   }

  string Value2=  TextBox2.Text;


 Con.Open();
  Cmd = new SqlCommand("Insert into t1 values (@Value1,@Value2)", Con);


  Cmd.Parameters.AddWithValue("@Value1", Value1);
  Cmd.Parameters.AddWithValue("@Value2", Value2);
  Cmd.ExecuteNonQuery();
  Con.Close();

You have to make sure your TextBox1.Text is a numeric value otherwise your SQL statement will give you an error as you are passing an empty string as a number.

The error is caused by your inline code

"insert into fir values("& TextBox1.Text &",' " & textBox2.Text & " ')"

You need to update this code to handle your given scenario.