Don't do it like that!
Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead.
When you convert that to parameterized queries, you validate and convert all your values in your C# code, and pass error messages back to your user when they have mistyped - you then pass the validated and converted values directly to SQL:
int iVal;
if (!int.TryParse(txtThisShouldBeANumber.Text, out iVal))
{
... Report input problem to user ...
return;
}
using (da = new SqlDataAdapter("INSERT INTO MyTable (MyColumnName) VALUES (@NUM)", com))
{
da.InsertCommand.Parameters.AddWithValue("@NUM", iVal);
...
}
It's more friendly for your users, easier to read, easier to maintain, and a damn site safer that passing control of your DB to the user! And as another advantage, your problem will disappear at the same time...
BTW: Do yourself a favour, and stop using Visual Studio default names for everything - you may remember that "TextBox8" is the mobile number today, but when you have to modify it in three weeks time, will you then? Use descriptive names - "tbMobileNo" for example - and your code becomes easier to read, more self documenting, easier to maintain - and surprisingly quicker to code because Intellisense can get to to "tbMobile" in three keystrokes, where "TextBox8" takes thinking about and 8 keystrokes...