Hi,
some where in your program you are trying to convert invalid decimal string to Decimal(i.e. " " or "abc" to decimal)
Please refer below code.
C#
TextBox3.Text == Convert.ToDecimal((decimal.TryParse(TextBox1.Text, null) ? TextBox1.Text : "0")) + Convert.ToDecimal((decimal.TryParse(TextBox2.Text, null) ? TextBox2.Text : "0"))
VB
TextBox3.Text = Convert.ToDecimal((If(Decimal.TryParse(TextBox1.Text, Nothing), TextBox1.Text, "0"))) + Convert.ToDecimal((If(Decimal.TryParse(TextBox2.Text, Nothing), TextBox2.Text, "0")))
I hope this will help you out.