How to prevent System.NullReferenceException

Question

I am trying to get data from form collection as:

foreach (string key in col.Keys)// where col is object of form collection
{
    if (col["ConstituntName[" + i + "]"].ToString() != null)
    { 
        UserRecordSubClass usr = new UserRecordSubClass();
        usr.Value = Convert.ToDecimal(col["ConstituntName[" + i + "]"]);
        usr.ConstituentNameId = Convert.ToInt32(col["ConstituntNameId[" + i + "]"]);
    }

    i++
}

but when the i is 2, ConstituntName["+i+"] does not exist so it throws:

System.NullReferenceException.

How can I prevent this exception in that case?

OT you are doing a `foreach (string key ..`, but are never using that `key` (in your posted code) — Hans Keﬆing, Dec 28 '19 at 13:15

score 2 · Accepted Answer · answered Dec 28 '19 at 13:12

2

Just check it for null before calling .ToString():

if ( col["ConstituntName[" + i + "]"] != null 
     && col["ConstituntName[" + i + "]"].ToString() != null)

answered Dec 28 '19 at 13:12

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score 0 · Answer 2 · answered Dec 28 '19 at 13:28

You could use the null conditional operator,

if (col["ConstituntName[" + i + "]"]?.ToString() != null)

The null conditional operator gets evaluated to null if col["ConstituntName[" + i + "]"] or col["ConstituntName[" + i + "]"].ToString() gets evaluated to null

How to prevent System.NullReferenceException

2 Answers2