validation of user input using custom validator with datagrid

thanx for the reply and help. well i am using custom validator with datagrid. i have the following code:

void datavalidate(Object s, ServerValidateEventArgs b)

{

string mark;

double mar;

mark=b.Value.ToUpper();

mar=Convert.ToDouble(b.Value);

if((mark=="A")||(mar<=50))

{
b.IsValid = true;
}

else

{
b.IsValid = false;
}
}


Here in my porgram user have to either enter the marks or A for absent.while compling i got following error:

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

waiting for the reply.


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