Empty textField

Hello friend's, i'm beginner in vb.net & i creating a billing project but i face some difficulties around it that i have separate 3 field for rate, quantity and amount for different product but when i left blank some field which i don't want so it's throw an exception 
Conversion from string "" to type 'Double' is not valid.

Dim Srate As Integer = cmbSrRate.Text Dim Sqty As Decimal = txtShrQty.Text Dim SAmount As Decimal SAmount = txtShrAmount.Text If String.IsNullOrEmpty(cmbptRate.Text And txtShrQty.Text And txtShrAmount.Text) Then Return Else txtShrAmount.Text = Srate * Sqty End If Dim Prate As Integer = cmbptRate.Text Dim Pqty As Decimal = txtPtQty.Text txtPtAmount.Text = Integer.Parse(Prate * Pqty)



so plz help me!
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