System.FormatException: 'Input string was not in a correct format.'

Convert.ToInt32(txtAge.Text)
failed because txtAge.Text is not a valid number.

int age;
if (!int.TryParse(txtAge.Text, out age))
{
    // Handle invalid input
    lblMessage.Text = "Please enter a valid number for age.";
    return;
}

SqlCommand cmd = new SqlCommand("INSERT INTO students (StudentName, Age) VALUES (@StudentName, @Age)", con);
cmd.Parameters.AddWithValue("@StudentName", txtName.Text);
cmd.Parameters.AddWithValue("@Age", age);

con.Open();
int rows = cmd.ExecuteNonQuery();
con.Close();
lblMessage.Text = "Record inserted successfully!";

Let’s break it down clearly 

 Error Message

System.FormatException: Input string was not in a correct format.

 Line 44:

cmd.Parameters.AddWithValue("@Age", Convert.ToInt32(txtAge.Text));

 Root Cause

This error means:

The value inside txtAge.Text cannot be converted to an integer using Convert.ToInt32().

 Common causes:

1. txtAge.Text is empty (e.g., "")

2. txtAge.Text contains non-numeric characters (like "twenty", "12 years", "12.5", or " " spaces)

3. The textbox value is null

4. The page is not validating input before inserting

 Fix: Validate Before Converting

You must ensure the input is numeric before calling Convert.ToInt32().

 Safe Fix:

int age;

if (int.TryParse(txtAge.Text.Trim(), out age))
{
    SqlCommand cmd = new SqlCommand("INSERT INTO students(StudentName, Age) VALUES (@StudentName, @Age)", con);
    cmd.Parameters.AddWithValue("@StudentName", txtName.Text.Trim());
    cmd.Parameters.AddWithValue("@Age", age);
    
    con.Open();
    int rows = cmd.ExecuteNonQuery();
    con.Close();
    
    lblMessage.Text = "Record inserted successfully!";
}
else
{
    lblMessage.Text = "Please enter a valid number for Age.";
}

 What int.TryParse() Does

• It tries to convert text to integer.

• If it fails, it does not throw an exception.

• It returns false — letting you show a user-friendly message instead.

 Bonus Tip — Input Validation

To prevent invalid input from the start, you can add an ASP.NET validator in your .aspx page:

 Summary

Cause	Fix
Empty textbox	Check for empty before converting
Non-numeric input	Use int.TryParse()
Missing validation	Add front-end validation
Null value	Initialize or validate input first

The errror is triggered on this line: cmd.Parameters.AddWithValue("@Age", Convert.ToInt32(txtAge.Text));
That means txtAge.Text is not a valid integer at runtime.
This can occur if:

•     The field is left blank.
•     The input contains non-numeric characters.
•     The number is outside the valid Int32 range.

So, before using, you should validate the input:

int age;
if (!int.TryParse(txtAge.Text, out age))
{
    // Handle invalid input (show message, default to 0, etc.)
    Response.Write("Please enter a valid number for age.");
    return;
}

But my recommendations is don’t use AddWithValue blindly. It can cause type mismatches. Use SqlDbType:

int age;
if (!int.TryParse(txtAge.Text, out age))
{
    // Handle invalid input (show message, default to 0, etc.)
    Response.Write("Please enter a valid number for age.");
    return;
}
cmd.Parameters.Add("@Age", SqlDbType.Int).Value = age;
cmd.Parameters.Add("@StudentName", SqlDbType.NVarChar, 100).Value = txtName.Text;