Object reference not set to an instance of an object [C#]

Question

I am getting the following error:

Object reference not set to an instance of an object

I don't know what is the cause of this error and how to solve it, here is my code:

while(dr.Read())
    {
     string variant = dr.GetString(0);
     int size = dr.GetInt32(1);
     int quantity = dr.GetInt32(2);

     DataRow x = dt.Rows
                  .Cast<DataRow>()
                  .Where(r => r["variant_name"].Equals(variant) && r["size"].Equals(size))
                  .FirstOrDefault();
                  x["quantity"] = quantity;             

      }

I'm getting the error on this line -> x["quantity"] = quantity; I'm wondering why it would give a null value because I checked in my database that it should return exactly one match.

`x` is null, so your `Where` clause is not yielding any results. — Lee, May 27 '13 at 12:21

score 3 · Answer 1 · answered May 27 '13 at 12:23

3

FirstOrDefault returns null if the element is not found and you try to access this object.

answered May 27 '13 at 12:23

Ken de Jong

219
1
11

Sir I already check my database and it should give at least one match. – Harvey May 27 '13 at 12:49
Try adding a breakpoint just before x["quantity"] = quantity; and check the value of x. I am pretty sure it is null when you get this error. – Ken de Jong May 27 '13 at 14:58
In visual studio, go to the line you want to break at and press F9 – Ken de Jong May 28 '13 at 07:29

Steve · Answer 2 · 2013-05-27T12:30:44.490

2

On the last line x is null, fix with

if(x != null)
   x["quantity"] = quantity;

This happens if your condition (.Where) doesn't yield a match.
At that point the FirstOrDefault return the Default part
and this is a null for a reference object as explained in MSDN

The default value for reference and nullable types is null.

edited May 27 '13 at 12:30

answered May 27 '13 at 12:22

Steve

213,761
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I check my database and it should give at least one match. – Harvey May 27 '13 at 12:31
Sorry Sir...I don't know what to change in that line of code. What do you think is wrong in that line `.Where`? – Harvey May 27 '13 at 12:48
No, I was thinking about a reference problem, but this is not the case. Are you sure that the string in your database field "variant_name" is written in the exact case of your test condition (no spaces or linefeed also) – Steve May 27 '13 at 12:56
If I understand you correctly, Yes I'm pretty sure that it is the same. Actually I've tried to check what is the value of the `variant` by using a messagebox and it returns the same. – Harvey May 27 '13 at 13:05
I have no explanation then, if you have a row with the exact variant_name and exact size this code should return a match – Steve May 27 '13 at 13:13

score 0 · Answer 3 · answered May 28 '13 at 02:17

while(dr.Read())
 {
  string variant = dr.GetString(0);
  int size = dr.GetInt32(1);
  int quantity = dr.GetInt32(2);

  DataRow x = dt.Rows
                .Cast<DataRow>()
                .Where(r => (r["variant_name"].ToString().Equals(variant) && Convert.ToInt32(r["size"]).Equals(size)))
                .FirstOrDefault();

   if (x != null)
   {
       x["quantity"] = quantity;  
   }       
 }

Thank you for your answer it helped me a lot. I just changed the line .Where a little bit and now it works for me. Thank you again!

Object reference not set to an instance of an object [C#]

3 Answers3