对象引用未设置为对象的实例[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.

Answer 1

如果找不到元素，并且您尝试访问此对象，则FirstOrDefault返回null。

Answer 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.

Answer 3

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!