对象引用未设置为View中对象的实例

Question

I have following code in view(in javascript)

 console.log("@Model.Sides.SingleOrDefault(a => a.Name == "B").Surfaces.Count");

it throws an exception

Object reference not set to an instance of an object

How to make that when an exception is thrown-nothing happened?

Answer 1

In your case there might be situation when there are no items in @Model.Sides In that case you can use.

@if(Model.Sides.Any() && Model.Sides.SingleOrDefault(a => a.Name == "B").Any())
{
console.log("@Model.Sides.SingleOrDefault(a => a.Name == "B").Surfaces.Count");
}
else
{
console.log("0");
}

Answer 2

You just need to check your objects are initialized first. This logic should really be outside of your views.

@{
    int count = 0;

    //Check the model isnt null first
    if (Model != null)
    {

        var side = Model.Sides.SingleOrDefault(a => a.Name == "B");

        if (side != null)
        {
            //You can perform null check on surfaces here as well
            count = side.Surfaces.Count;
        }

    }


}

<script>

    console.log("@count");

</script>

Answer 3

You can try the following

@if(Model.Sides.SingleOrDefault(a => a.Name == "B").Any())
{
console.log("@Model.Sides.SingleOrDefault(a => a.Name == "B").Surfaces.Count");
}
else
{
console.log("0");
}

Answer 4

SingleOrDefault will return a null if the list does not contain exactly one item.

If your 'where' clause a.Name == 'B' does not return exactly one item then the result will be null.

So if your Sides list does not have just one 'B' (ie there are none or 2 or more than 2), the result of SingleOrDefault will be null and .Surfaces will give the error 'Object reference not set to an instance of an object`.

If you simply want a count of zero when there isn't just one B side, then use:

console.log("@(Model.Sides.SingleOrDefault(a => a.Name == "B") ?? new Side { Surfaces = new List<Surface>() }).Surfaces.Count");

where ?? means: if the left part is null, return this instead of null and you create a dummy Side with an empty list so that Surfaces.Count == 0

Alternatively, use a variable:

@{
    var side = Model.Sides.SingleOrDefault(a => a.Name == "B");
    if (side == null) 
    {
        @:console.log("none")
    } 
    else 
    {
        @:console.log(@side.Surfaces.Count);
    }
}

As a cleaner alternative, you could use SelectMany() instead, something like (untested) :

Model.Sides.Where(a=>a.Name=="B").SelectMany(x=>x.Surfaces).Count()

then you don't need to worry about temp variables etc but will return the number of 'surfaces' for all 'B' sides, so may not be your requirement.

Answer 5

On Catch(e) do nothing:

This is generic that can suppress any error. Wrap the code you think can error with try catch, with catch doing nothing.

But this could lead into further problems, just be cautious with empty catch block.

ex:

try
  {
       //code that errors
  }

catch (e)
  {
      ;
  }

//Continue