Try This
SqlCommand command = new SqlCommand("INSERT INTO dbo.Form(Name,Email,Mobile,Country) VALUES ('" + txtname.Text + "','" + txtmail.Text + "', " + txtmobile.Text + ",'" + ddlcountry.SelectedItem + "') ",connection);
In above statement, My assumption is 'Mobile' column is Integer.
Other wise
SqlCommand command = new SqlCommand("INSERT INTO dbo.Form(Name,Email,Mobile,Country) VALUES ('" + txtname.Text + "','" + txtmail.Text + "', '" + txtmobile.Text + "','" + ddlcountry.SelectedItem + "') ",connection);
You have to use like this. For string values
'"+ value1 +"'
, integer values
"+value2 +"
.
Note: Have a look on my Tip
Formatting a string[
^]