Check if a value is null before casting and assigning

Question

I'm using c# .net 2.0 and need to check if the value returned from my method that I am assigning my object to is null.

My code is

MyObjectValue myObjectValue= (MyObjectValue) myObjectField.GetFieldValue();

In this instance the value returned by myObjectField.GetFieldValue() could be null and I want to check this before assigning to myObjectValue. At the moment it throws an exception object reference not set to a value of an object.

The actual line of code is using the SharePoint API

SPFieldUserValue lawyerResponsibleFieldValue = 
    (SPFieldUserValue)lawyerResponsibleUserField.GetFieldValue(
              workflowProperties.Item[lawyerResponsibleUserField.Id].ToString());

Obivously an assignment can't throw, so your problem is that `myObjectField` is null, not the return value or something within the `GetFieldValue` method. — John Leidegren, Jan 26 '11 at 18:30
@John: Or the cast fails, which it would if `MyObjectValue` is a value type. — Adam Robinson, Jan 26 '11 at 18:34
That actual line of code is almost *nothing* like the original. You've introduced many, many potential causes of a NullReferenceException. See my edit. — Jon Skeet, Jan 26 '11 at 18:35
Good grief, that's a heck of a line of code. You need to decompose it; test each value for null, to see where the problem is. — Michael Petrotta, Jan 26 '11 at 18:35
List of possible causes of NullReferenceExceptions: http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-in-net/4660186#4660186 — Justin, Jan 26 '11 at 18:43

Jon Skeet · Accepted Answer · 2011-01-26T18:35:09.743

6

The above code won't throw a NullReferenceException if myObjectField itself is non-null, unless MyObjectValue is a value type. Are you sure the problem isn't that you're then using myObjectValue without checking whether or not it's null?

However, assuming GetFieldValue returns object, the simplest approach is simply to use a temporary variable:

object tmp = myObjectField.GetFieldValue();
if (tmp != null)
{
    MyObjectValue myObjectValue = (MyObjectValue) tmp;
    // Use myObjectValue here
}

Obviously this will work whether MyObjectValue is a reference type or a value type.

EDIT: Now that you've posted the full line of code, there are loads of places where it could be throwing a NullReferenceException. I strongly suggest that you break up that one line into several lines to find out what's going on.

edited Jan 26 '11 at 18:35

answered Jan 26 '11 at 18:28

Jon Skeet

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@nav: Is "MyObjectValue" a value type or a reference type? – Jon Skeet Jan 26 '11 at 18:30
from his edit, `MyObjectValue` is actually `SPFieldUserValue`, a reference type. – Justin Jan 26 '11 at 18:34
@Justin: Yup, and the edit completely changes the question :) – Jon Skeet Jan 26 '11 at 18:35

score 5 · Answer 2 · answered Feb 23 '17 at 03:09

5

C# 6 (Visual Studio 2015) introduced Null-Conditionals (aka Elvis operator)...

instead of this...

myObjectValue = (myObjectValue == null) ? null : myObjectValue.child;

the following can be used...

myObjectValue = myObjectValue?.child;

answered Feb 23 '17 at 03:09

SunsetQuest

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John Leidegren · Answer 3 · 2011-01-27T05:24:26.637

Anywhere you . something, it represents member access, if the left-hand side is a reference type (class) it could be null.

This snippet has several places where a null value might occur:

workflowProperties.Item[lawyerResponsibleUserField.Id].ToString()
                  ^                               ^   ^

I would first of all, break into a debugging session and check what values are present and read up on any documentation regarding the issue. Then I might reconsider writing the code like this.

if (lawyerResponsibleUserField != null
    && lawyerResponsibleUserField.Id != null)
{
    return Convert.ToString(workflowProperties != null 
            ? workflowProperties.Item[lawyerResponsibleUserField.Id] 
            : null
    )
}

Assuming that these properties are reference types and could be null values.

There are many ways to check for null and short circuit the evaluation. A good tip is to avoid using the .ToString() instance method and use the static method Convert.ToString() instead, it doesn't throw an exception if the passed value is null. The return value can still be null, but at least you don't have to worry about the supplied argument as much.

score 1 · Answer 4 · answered May 21 '16 at 08:37

Bellow suggestion is just a sample of null values

if object is null then object.ToString() will raise error

in bellow codes i use ComboBox for samples.

so best way is check that

// 1)
if (cmb.SelectedValue == null)
{
   // do something
}

and if you want to use inline

// 2)
string strValue = (cmb.SelectedValue ?? "-1").ToString();
// or
string strValue = cmb.SelectedValue == null ? "-1" : cmb.SelectedValue.ToString();

and last way is to handle error

// 3)
string strValue = "";
try
{
   strValue = cmb.SelectedValue.ToString();
}
catch
{
  strValue = "-1";
}

score 1 · Answer 5 · answered Jan 26 '11 at 18:29

1

If you want to be really safe:

MyObjectValue myObjectValue = null;

if(myObjectField != null)
{
    object temp = myObjectField.GetFieldValue();
    if(temp != null)
    {
        myObjectValue = (MyObjectValue)temp;
    }
}

answered Jan 26 '11 at 18:29

Justin Niessner

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`(cast)null` is okay and results in `null` -- I "take advantage of" this a good bit. Sometimes it's for supplying type information to a method as well with type-inference, etc. Only the outside conditional is useful here. Example: `T Coerce(object val, T defaultValue) { ... }; string s = Coerce(fuuba, (string)null);` – Jan 26 '11 at 18:49
Previous comment doesn't address ValueTypes. Leaving for history, but "really safe" does apply without knowing type of MyObjectValue more. – Jan 26 '11 at 19:03

score 1 · Answer 6 · answered Jan 26 '11 at 18:30

I have a feeling that it's actually myObjectField that's null, not myObjectField.GetFieldValue(). If that's the case, then a simple check for null would work fine:

 MyObjectValue myObjectValue;
if  (myObjectField == null)
{
    // recover
}
else 
{
    myObjectValue = (MyObjectValue) myObjectField.GetFieldValue();
}

A few lines of debug code would verify that for you:

Console.WriteLine("myObjectField=" + myObjectField);

If you see null there, then it's myObjectField that's null.

score 1 · Answer 7 · answered Jan 26 '11 at 18:30

1

You may have an error elsewhere; if MyObjectValue is a reference type (a class, not a struct), then there should be no exception when casting null to your type. If MyObjectValue is a value type (a struct, not a class), then you cannot assign a null value to it.

You can use the coalescing operator to handle the case for nulls.

MyObjectValue value = (MyObjectValue)(field.GetFieldValue() ?? (object)someValidValue);

someValidValue represents what you'd like to use as the default value in the case that GetFieldValue returns null. This is slightly expensive, since casting someValidValue must be boxed to cast it to an object, but this should do what you're looking for.

answered Jan 26 '11 at 18:30

Adam Robinson

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Does no better. `(cast)null` is okay. Problem is `field` may be null. Unless I'm suffering from the question-edit bug. Might as well be `((MyObjectValue)field.GetFieldValue()) ?? someMyObjectValue` wrt. a NRE. – Jan 26 '11 at 18:55
Oops on me. +1 because this does address when MyObjectValue is a ValueType. Other comment left for history. – Jan 26 '11 at 19:04

score 0 · Answer 8 · answered Sep 25 '18 at 13:45

0

This should work.

if (myObjectField  != System.DBNull.Value))
{
    string variable = myObjectField.ToString();
}

answered Sep 25 '18 at 13:45

starko

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score 0 · Answer 9 · answered Feb 12 '19 at 22:26

0

 public IQueryable GetOrderDetails()
    { 
        var _db = new ProductContext();
        IQueryable<OrderDetail> query = _db.OrderDetails;

        if (ddlOrder.SelectedValue != null && ddlOrder.SelectedItem != null)
        {
            var id = Convert.ToInt32(ddlOrder.SelectedValue);

            query = query.Where(p => p.Username == User.Identity.Name && p.OrderId == id);
                }   
       else{ query = null; ddlOrder.Visible = false;btnSubmit.Visible = false; }
        return query;
           }

answered Feb 12 '19 at 22:26

Automata company

1

It would be helpful for @Van (and future users) to hear your explanation as to why this is the right answer. – Josh Gust Feb 12 '19 at 22:43
If Drop down value is null or selected item is null or no data in dropdown list then we can use it. if (ddlOrder.SelectedValue != null && ddlOrder.SelectedItem != null) – Automata company Feb 14 '19 at 11:04

Check if a value is null before casting and assigning

9 Answers9