Convert string to integer without predefine method in c#

private void btnconvert_Click(object sender, System.EventArgs e)
{
string Svalue=null;
int i=0, sum=0;
Svalue=txtstringvalue.Text;
Svalue=Svalue+'\0';
while(Svalue[i]!='\0')
{
if(Svalue[i]< 48 ||Svalue[i] > 57)
{
lblintegervalue.Text="Invalid value !";
return ;
}
else
{
sum=sum*10+(Svalue[i]-48);
}
i++;
}
lblintegervalue.Text=Convert.ToString(sum);
MessageBox.Show("Integer value is :"+sum);
}