convert hiddenfield datatype

before submit the form, the string is "aaa"
after submit the form, the string always be null (initialized)
how can i get the string value "aaa" after submit the form? 
I already used the hidden field to store the value, it seems fine, but i can't convert the type of value to original type (from string to int)
before submit the form i use the following code:
int ori_val=1;
hiddenfield.value=ori_val.toString();   <-- convert to string and store to hidden field
after submit form ( i want to convert to original type int)
int val = Int32.Parse(hiddenfield.value);
int.Parse(hiddenfield.Value);
 
all of the above methods have the same error: "input format not correct"
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