Null Reference Exception in a Dynamic LINQ Expression



I am using the Dynamic Linq Library / Sample from Microsoft to do ordering on a list. So for example I have the following C# code:

         .OrderBy(sortField + " " + sortDirection).ToList();

I have a case where my Object have a 0:1 relationship with another object, which has a property that might be displayed in the grid. When we try and sort this, it works fine so long as all my Widgets have this child. We are ordering by Child.Name for example. When Child is null however, we get the null reference exception.

I have some options here which I know I could select into an anonymous type and bind to that, I could also expose the Child.Name on the parent object and handle this via code (Which I don't like comprising my object model for this).

In an ideal world I'd like to update the library to handle this case. Before I dive into it, I'm wondering if anyone has ran across this or not and has a solution already?


Looks like I didn't explain well enough. I am using the Dynamic Linq Library which comes with the C# samples. This library adds some nice extensions that let you use a string inplace of a lambda expression So my code is actually something like this:

private  void BindGrid(sortField,sortDirection)

     this.grid.DataSource=....OrderBy("MyField ASC")....

Of course the string there is replaced with the parameters. But this allows us to change the sorting dynamically as the user clicks on a grid header. We don't have to if then else logic to handle all the permutations.

My solution as I documented bellow changes my nice clean method into:

private void BindGrid()
   var sortField=this._sortField;
   if (sortField=="Child.Name")
                                  .OrderBy(sortField + " " + this._sortDirection)

And while this works, this now means I have to update this code as we add new fields or properties which we want to expose in the grid which are on a child object.

4 个答案



    .OrderBy(p => p.Child == null ? "" : p.Child.Name);


  • Nope this is not what I am referring to. The dynamic linq is part of the C# sample code and lets you use a string as an OrderBy for example. It compiles the string to a lambda expression at run time.

    JoshBerkeSeptember 18, 2009 16:37


     .OrderBy(w => w.SortField).ToList();

其中SortField是要排序的属性。 即使该值为null,它也应该起作用。


  • You completly missed the point. Since your not the only I must have not explained myself well enough. I updated the question so it is probally a little but more clear.

    JoshBerkeSeptember 18, 2009 16:46


 iif(Child == null,null,Child.Name) ASC




 private void BindGrid()
    var sortField = this._sortField;
    var splitted_sortField = this._sortField.Split(new char[]{'.'}, StringSplitOptions.RemoveEmptyEntries);
    if (splitted_sortField.Length > 1)
        sortField = "iif("+splitted_sortField[0]+"==null,null,"+sortField+")";
    this.grid.DataSource = repository.GetWidgetList()
                                     .OrderBy(sortField + " " + this._sortDirection)